New: Add u and y regex flags (refs #10)

Looking for some feedback.

The tests are failing because I had to add the missing `regex` property to both AST nodes and tokens when they represent a regular expression. This is because `value` is unreliable given that Node.js doesn't support either flag, meaning we can't create a new `RegExp` to assign to `value`. The `regex` property is already in Esprima Harmony, I just ported it over.

The failing tests are those of the third-party JS libraries that Esprima uses to test AST output. I haven't had the time to convert those as they are massive. 

Also, I found a bug where `tokenize()` wasn't respecting the `ecmascript` setting. That's fixed in this commit.