Unify indent style

Add Fiber's double resume test and fix it.

Fix Enumerator

Don't redefine timeval in MinGW environments

Fix compile error check of Mrbc command.

add MRB_INT_BIT

add including limits.h.

add mrb_strlen_lit

Because Android's libc(bionic) defines SIZE_MAX at limits.h.

strlen(3) + string literal is usually optimized but strlen(3) doesn't
check if its argument is really a string literal. This is important for
mruby's _static functions to which some _lit macros are expanded (string
literals have static storage). See comment for some additional info.

remove unnecessary parentheses

ignore *.pdb

Added a short description about mruby gems