Documentation generator script and some readme updates

Documentation generation solution is borrowed from Ecto as suggested by @ericmj.

You can check documentation at http://phoenixframework.github.io/docs/. For now it's just in repository root. One day we will have master and stable and after that we can introduce version directories, but lets keep it simple for now.

There are also some Readme updates. It's still very rough but we will get there.