Lexer for the Go programming language

As an answer to [Issue 60](https://github.com/jayferd/rouge/issues/60), I implemented a lexer for Go.

It highlights tokens as well as escape sequences in strings. I followed the [language specification](http://golang.org/ref/spec) closely so hopefully I did not forget anything.

I did not implement more advanced parts (yet!) such as highlighting exported identifiers or function prototypes, but I believe the current lexer still makes Go code easier to read and lays down foundations for future enhancements.