Provide an asynchronous API

[rjmholt]

PSSA2 should provide an API set that allows hosts to run PSSA asynchronously.

This includes:

• A set of asynchronous methods to run analysis, with cancellation token support
• A new rule executor that provides an asynchronous run strategy, including for non-threadsafe rules
• An asynchronous rule API for rules wishing to implement their own asynchronous methods
  • It should be possible to implement an asynchronous and synchronous rule in the same class
• An asynchronous harness for running synchronous rules with a minimum of overhead