Add snippets area to docs.

[rduplain]

Some code is better left to well-understood snippets than parameterized library code.

[rduplain]

We want a WillowTree snippets repository instead of snippets in the docs. Some iOS guys use CodeBox.
http://www.codeboxapp.com/

Here's a simple implementation from the Flask project:
http://flask.pocoo.org/snippets/

[rduplain]

We can always use gist, too, but we'd need an aggregator.