Added unit tests for sensu 0.13 + fixed sensu 0.13 filtering

Unit tests for sensu 0.13 have been added (with new fixtures).

Issues with the filtering functions have been also fixed to match the new JSON event data structure.

It's now also possible to match numbers as well as strings.
is `function?` a function which returns a boolean value? I'd expect such a function to be named `pred`, to be consistent with core documentation (eg have a look at `(doc filter)`).