Add build on Trusty and OS X

The default 6.1 was slow

be able to source with :path for mrbgems

pop cmdarg in lambda body; fix [ruby-bug#11380]

regression introduced by 2fe556d9

mrbgem compile should be depend on mrbgem.rake

Shallow clone mgem source

update mrbgems doc

fix android compile bug : uninitialized constant GCC_VERSION

Add ability to change compiler versions for androideabi

Fix indents; Indent is two spaces; Delete tabs [skip ci]

Added documentation for hash.

Don't crash if pattern not found for sub

64bit Cygwin also doesn't have frexpl(3)

use mrb_str_cat_str() instead of mrb_str_append()

delete mrb_free()-related non-NULL checks

No need to optimize since a program only exits once and errors are rare.
Also the mruby source code doesn't have these kind of checks elsewhere.

The ones in {Time,Random}#initialize are kept because there it actually
matters
since initialization always happens and re-initialization is unlikely.

If the argument is always a string, then mrb_str_cat_str() can be
directly called instead of indirectly by mrb_str_append().

mrb_any_to_s(), mrb_obj_as_string(), mrb_inspect() always return a
string.