Swift syntax highlighting

Extended Flatdoc's highlighting library to include support for swift language. Refer to screenshot attached. Color scheme is based on the Appl's Swift iBook syntax.

![screen shot 2014-06-10 at 5 12 05 pm](https://cloud.githubusercontent.com/assets/1315101/3227792/61085014-f07f-11e3-9566-834f46221387.png)