Accept-Language HTTP header parsing handler

I've added a `martini.Handler` implementation that parses the `Accept-Language` HTTP header and exposes it as an `AcceptLanguages` struct. The implementation is basically copied from the [contribution I made to Revel](https://github.com/robfig/revel/pull/61) several months ago.

It's fully tested, and I have added 2 benchmarks in case anyone wants to have a go at finding an more efficient implementation.

Let me know what you think.