Add block binding (let/const) support

fixes #65
+ tests
Enable native support for let/const in node v0.11.2+.
Merge pull request #1 from facebook/issue65

Enable native support for let/const in node v0.11.2+
Add block bind (let/const) support 2
Merge pull request #67 from termi/issue65

Add block binding (let/const) support.