Convert comment-documentation to use Docile

I did two things:
1. Add Docile to the REQUIRE file.
2. Convert HttpServer.jl to use `@doc` strings, rather than comments for documentation.

A couple questions:
* Are you up for using Docile.jl to document this, or do you want to wait for such a thing to appear in Base?
* Do you want to use Lexicon.jl? (Running `julia -e 'using Lexicon, HttpServer; save("doc/site/master/index.html", HttpServer)'` inside the `HttpServer` directory will get you the autogenerated HTML file.)