feat(routing): New router architecture and faster default router

[richardolsson]

This replaces the routing logic with a new, faster strategy based on on-the-fly
generation of Python code. As part of replacing the logic, the architecture has
been refactored to decouple routing from falcon.API, thereby making it possible
for users to easily replace the default router with their own.

Closes #114

[richardolsson]

Unfortunately the distributed benchmarks don't show any performance gain for this code compared to previous versions, because the test application only uses a single route. However, my experiments and benchmarks in a separate repository shows (pure-routing) speed gains of anything between 2x and 100x in real-life conditions.

[richardolsson]

Replacing this with a new pull request which will pass refined PEP8 requirements.