Add support for connecting to Full Agent via WebSocket

[dbluhm]

Currently, the Static Agent Library only has support for sending messages over POST to its paired Full Agent. This covers one of the most common use cases. Supporting WebSockets will enhance communications where multiple messages are going to be sent and received.

StaticAgentConnection should determine whether to use POST or WebSocket by checking the schema of the endpoint passed to it (http://example.com/ vs. ws://example.com/).

This is closely related to #5 since the paired Full Agent won't respond over the current connection unless told to do so through the ~transport decorator and a specified return_route.