Added MRUBY_LIBS to mrbgems Makefile
Support MakefileGemList for MRUBY_CFLAGS and MRUBY_LDFLAGS

Other apps using mruby like mod_mruby easy to link mrbgems dynamically by MakefileGemList
add mrb_mod_cv_defined and mrb_cv_defined
add mrb_mod_cvar_defined
add test for Module#class_variable_defined?
Support GEM_LIBS_LIST in Makefile4gem
add mrb_mod_remove_cvar
add test for Module#remove_class_variable
replace all slash to backslash in `cd /path/to/folder` for windows.
use readline if it's enabled.
wrong stack position for headless case; close #675
fix correct stack postion for headless case
minor cosmetic change after #676
add mrb_mod_remove_method/remove_method
add test for Module#remove_method
Fix a wrong comment
Rebuild CRuby based building script without Makefile

Tested CRuby 1.8.6 and 1.9.3

You can see building configuration in build_config.rb
Adapter mrbgems documentation to rake system
Fix documentation for mrbgems
Add documentation for compilation process