Add pat muxer with parameters, stub routes

* Register handlers on a pat.Router instead of DefaultServeMux
* Add stub routes for note browsing, login flow, and the note API
* Use pat's variable pattern parameters and HTTP verb matchers
Refactor gopherpad executable as a project service

* Split gopherpad executable into a library package and executable
* Make gopherpad a component (service) in the Gopherpad project
* Update README on running the app stand-alone
Add Sphinx docs and intro.rst, add .gitignore
Add minimal-server docs

* minimal-server docs show a hello world example and basic routing
* Add docs _static sources for images
* Use '*', '=', '-' for top level section title markup
Change docs theme from default to alabaster

* Add docs/requirements.txt for use by ReadTheDocs
* Add RTD docs badge to the README
Add docs on muxers with pattern parameters