Add check for using cdump with DISABLE_STDIO option.

Export Array#clear in array.h

Added gem :github option

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