Fix misspellings in document

Add rake task for benchmarking

Use builtins for overflow math if possible

Suppress warnings generated by -Wwrite-strings

Here are suppressed warnings:

    src/fmt_fp.c: In function 'fmt_fp':
    src/fmt_fp.c:124:16: warning: initialization discards 'const' qualifier from pointer target type
         char *ss = (t&32)?"inf":"INF";
                    ^
    src/fmt_fp.c:125:17: warning: assignment discards 'const' qualifier from pointer target type
         if (y!=y) ss=(t&32)?"nan":"NAN";
                     ^
    mrbgems/mruby-string-ext/src/string.c: In function 'mrb_str_succ_bang':
    mrbgems/mruby-string-ext/src/string.c:302:27: warning: assignment discards 'const' qualifier from pointer target type
           if (e == b) prepend = "1";
                               ^
    mrbgems/mruby-string-ext/src/string.c:305:27: warning: assignment discards 'const' qualifier from pointer target type
           if (e == b) prepend = "a";
                               ^
    mrbgems/mruby-string-ext/src/string.c:308:27: warning: assignment discards 'const' qualifier from pointer target type
           if (e == b) prepend = "A";
                               ^
    mrbgems/mruby-bin-mruby/tools/mruby/mruby.c: In function 'main':
    mrbgems/mruby-bin-mruby/tools/mruby/mruby.c:213:13: warning: assignment discards 'const' qualifier from pointer target type
         cmdline = args.cmdline ? args.cmdline : "-";
                 ^
    mrbgems/mruby-bin-debugger/tools/mrdb/cmdbreak.c: In function 'print_breakpoint':
    mrbgems/mruby-bin-debugger/tools/mrdb/cmdbreak.c:159:3: warning: initialization discards 'const' qualifier from pointer target type
       char* enable_letter[] = {BREAK_INFO_MSG_DISABLE, BREAK_INFO_MSG_ENABLE};
       ^
    mrbgems/mruby-bin-debugger/tools/mrdb/cmdbreak.c:159:3: warning: initialization discards 'const' qualifier from pointer target type

Fix to avoid warning

Add test of negative arguments for Integer#%

fix maximum value of the index when access breakpoints.

remove unnecessary <inttypes.h> inclusion

MSVC 2015 implements inline keyword

The format specifier macros were needed to portably print a mrb_int,
because mrb_raisef() originally called vsnprintf(). It doesn't anymore
since 18b2683b and the mrb_int format
specifier macros are already gone.

Apparently the C compiler of Visual Studio 2015 CTP6 finally implements
inline as inline and not only as _inline and __inline.

mruby-time: call ISO C11's timespec_get() if available

refactor write_pool_block()

No need to write the same assertion in each case (except the default
one). Instead we can assert after the switch statement.

gettimeofday() is an obsolescent POSIX function which may be removed in
a future version. POSIX recommends using clock_gettime() (also POSIX)
instead, but it isn't available on OS X or Windows (at least with MSVC
and older MinGW versions).

Whereas timespec_get() is part of ISO C11 and mruby uses some small
other C11 features too. It isn't universally available yet either, but
it might be in the future. And Visual C++ 2015 implements it! Since
mruby strives for ISO C and not POSIX compatibility, I think it's a
reasonable choice.

TIME_UTC is used instead of __STDC_VERSION__, because if TIME_UTC is
defined, then most likely timespec_get() is too. This isn't true in case
of  __STDC_VERSION__ (see MSVC).

Fix exceptions for visualcpp