Compact flags for GC

Add test cases for GC API

Skip sweeping old slots which don't contain any young object in the minor GC

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