MessagePack 0.6 Spec

This pull request adds support for MessagePack 0.6 Spec and other features:
 - import 0.6 branch sources
 - NSString serializes to MessagePack str
 - NSData serializes to MessagePack bin
 - add support for direct serialization of NSString, NSNumber, NSData & NSNull
 - add Xcode project with targets for Cocoa Framework, dynamic lib and static lib
 - add unit test suite for checking that the complete 0.6 spec is supported

TODO: add unit tests for streaming parser