Pluggable results reporting

[vinceatbluelabs]

Idea came from @joearasin - Joe, maybe you could go into some more detail? One part would be providing a plugin module for posting the details of a run, and whether the result is a pass or fail.

[joearasin]

The basic thing I'm going for is that there's a bit of a seam for data storage -- the current system hardcodes "files" in metrics, but providing an interface that consists of read_target(testname) and ratchet(testname, value) would allow for pluggable storage backends.

[joearasin]

maybe even a one-method try_to_ratchet(testname, value), which throws if value > old_value works as the interface.