Update example.yaml and add tests

[PeterJCLaw]

This updates the example.yaml to correct its interactions with feeds, its next-state destination specifications and adds some tests that the example is valid and loads.

[coveralls]

Coverage remained the same at 100.0% when pulling f667acd on update-example.yaml into 7c6c1c4 on master.

[PeterJCLaw]

@danpalmer I've created a test that this loads and is valid, yet it's failing. When loading the config we appear to require that a state's next mapping contains a 'default' key when the next has a destinations, yet that doesn't match that schema. Does that reflect a bug in the loading code or the schema?

Currently the error that users see is a KeyError: 'default', so even if we can't actually express this in the schema I suggest that we output a nicer error.

[PeterJCLaw]

I've raised the destinations/default quandary as #50 and will just make the example work for the moment.