Allow globals that start with $_

Cleanup includes

Rearrange SIZE_MAX. It is supported also VC++ since its version10. And there seems SIZE_MAX is defined in stdint.h. And it possibly (depends on the version of VC++) conflicts with SIZE_MAX in limits.h. This patch is no need if I did not support VC++.

Fixed wrong section size calculation

Fix a type mismatch.

LEGAL File Generator for Builds

Add arena guard. It will be exausted when the pool is huge.

Use mrb_format() instead of s*printf().

Fix raisef() format strings in struct.c.

Fix format for new mrb_raisef().

Added debug infomation section into .mrb file

Add travis_config.rb

Use mrb_check_datatype directly to avoid warning.

use new mrb_format API from mrb_raisef; its only format specifier is "%S" (stringify) and takes mrb_value; close #1062

Add simple usage message for mirb