Added docs and extensible WSGI server supoort

I apologize this pull request is big mess. It combines docs + WSGI support .

To look for WSGI changes look into following commits.

- [1fa46f2efaf94710b5a825a4eb18733091a3b2ec](https://github.com/kracekumar/brubeck/commit/1fa46f2efaf94710b5a825a4eb18733091a3b2ec)
- [bd3d969972273bd4670f89ea53f100a90aff4604](https://github.com/kracekumar/brubeck/commit/bd3d969972273bd4670f89ea53f100a90aff4604)
- [a575d59857169a7c36b3744d810d8cd86748a466](https://github.com/kracekumar/brubeck/commit/a575d59857169a7c36b3744d810d8cd86748a466)
- [4b706949931b4167bd255748ddb46720132a1d5d](https://github.com/kracekumar/brubeck/commit/4b706949931b4167bd255748ddb46720132a1d5d)

Added `__call__` method to `Brubeck`.

I don't expect this pull request to be merged, but I take this as chance to discuss about better WSGI support with/without concurrency. 

Currently I haven't removed `run_forever_ever` for backward compatibility.