XQuery syntax highlighting improvements

This branch contains major improvements in the XQuery syntax highlighting.

The lexer is based directly from the XQuery specification and supports advanced tokenization such as string interpolation.
![Screen Shot 2013-01-25 at 12 50 29 PM](https://f.cloud.github.com/assets/306134/97205/76dfcc3e-66e5-11e2-8797-b984d4c95971.png)

The background semantic highlighter takes care of elements that cannot be computed during the lexical analysis. It support a caching mechanism of the semantic token in order to prevent the lexical (synchronous) and semantic (asynchronous) highlighting to clash between each other.
The clip at https://dl.dropbox.com/u/1487285/before.mov shows the synchronous and async highlighter clashing with one another. The clip at https://dl.dropbox.com/u/1487285/after.mov shows the current behavior.