Add ECMAScript 6 features

[nzakas]

This is a rollup that will let us keep track of ES6 progress.

• let declarations
• const declarations
• Regular expression u flag
• Regular expression y flag
• Binary literals
• New octal literals
• Arrow functions
• Default parameters
• Rest parameters
• Spread operator
• super references
• Classes
• for-of loops
• Generators
• Object literal property shorthand
• Object literal method shorthand
• Object literal computed properties
• Template strings
• Destructuring
• Unicode code point escapes
• Allow duplicate object literal properties (except __proto__)
• Modules

[nzakas]

The let and const changes are in. Next up: regular expression flags.

[nzakas]

In order to make the regular expression flags work, I needed to add a regex property to both AST nodes and literal nodes to coincide with Esprima Harmony. I've got this working, but need to update the AST tests for the various third-party libraries that Esprima uses to validate AST output.