Add MRB_SET_INSTANCE_TT() to classes that uses struct RData

This avoids memory leaks caused by an instance is collected as a
struct RObject.
Fix SEGV bug and minor fixes
Add PP::Core

Now works APIs related to time (get_time and get_time_ticks). Threads
APIs (call_on_main_thread and is_main_thread) are not implemented
because nacl-mruby doesn't currently support threading.
Add PP::Fullscreen
Add core gems and edit mruby-ppapi/mrbgem.rake
Add repl example
Edit README.md