Added cxx/objc/asmflags in mrbgems. Sorry, I forgot it
Modify NODE_IF handling.

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.
`unless` without `else` should return nil if the condition is true.
Removed XCode build
Remove unused rules in .gitignore and Makefile
Merge pull request #692 from masuidrive/fixed_mrbgem_cxx_objc_flags

Added cxx/objc/asmflags in mrbgems
Merge pull request #695 from masuidrive/refactoring_gitignore

Remove unused rules in .gitignore and Makefile
Merge pull request #694 from masuidrive/remove_xcode

Removed XCode build
Merge pull request #693 from masamitsu-murase/modify_unless

Modify NODE_IF handling.
These places need the write barrier
Add the generational gc
Merge pull request #696 from authorNari/gengc

Generational Garbage Collection for mruby
Run mrbgems test run on isolate mrb_state each rb file.

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.
Merge pull request #698 from masuidrive/improve_mrbgems_test

improve mrbgems
Remove DEBUG macro definitions.
gc.c: Enable DEBUG(x) if GC_DEBUG is defined.
vm.c: Enable DEBUG(x) if VM_DEBUG is defined.
add test cases for GC API
Compact flags for GC
Skip sweeping old slots which don't contain any young object.
Merge pull request #701 from monaka/pr-remove-debug-macros

Remove debug macros from mruby.h