Implement filters and middleware.

Implement filters and wrappers for Sencha Connect middleware.

Register will now run through all the URLs matched by Avenue/Locate and
stopping at the first one that raises and error or sends headers.

Register provides a middleware function that will accept any number of
Sencha Connect middleware functions and a callback. It will run through
the middleware functions, stopping when the middleware function either
raises and error or sends headers.

Closes #29.