Merge pull request #7 from lazytiger/master

add ArrayBufferAllocator support
Merge pull request #8 from lazytiger/master

fix nil callback issue
Merge pull request #9 from lazytiger/master

Add MessageListener. Refactor SetAccessor to keep rule as SetXXXPropertyHandler
Merge pull request #10 from lazytiger/master

Fix Allocator crush problem
Merge pull request #11 from lazytiger/master

add more test code for MessageListener
add MessageListener related function and test
add PrivateData support to Engine and Context to provide flexibatility
Merge pull request #12 from lazytiger/master

InternalField support
add GetEngine to ContextScope