Remove debug macros from mruby.h

vm.c: Enable DEBUG(x) if VM_DEBUG is defined.

improve mrbgems

Can access to current gem's name through GEMNAME from test.

Can change assert library each mrbgems. used spec.test_preload on mrbgem.rake.

assert library should set $ok_test, $ko_test, $kill_test, $asserts.

Generational Garbage Collection for mruby

Modify NODE_IF handling.

Removed XCode build

Remove unused rules in .gitignore and Makefile

Added cxx/objc/asmflags in mrbgems

When "unless" is used without "else", tree->cdr->car of NODE_IF is NULL.
In this case, pop() should not be called because codegen generates no code.

Fixed build dependency

Add mruby_ldflags when tools/mruby and tools/mrbc link archive files

Fixed some mrbgems building issues

Added cxxflags, objcflags, asmflags to mrbgem.rake
Support .cxx and .cc as C++ source in mrbgems

Add Module#method_defined? for mruby

Fix an unnecessary space

Add documentation for compilation process

Fix documentation for mrbgems

Adapter mrbgems documentation to rake system

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

Fix a wrong comment