Convert behat fixtures to use PHPUnit Datasets

Put this together to try and make our tests a little easier to write and more reliable.

All the fixtures data is here: https://github.com/ushahidi/Lamu/blob/aca1529533baf6e075031f5ba419136d18d2a05c/application/tests/datasets/ushahidi/Base.yml

The db gets rebuilt for each feature, and selectively for some scenarios..
I've added a @resetFixture tag you can add to scenarios to force the fixtures to rerun for that scenario. This is particularly useful for tests that search or list all resources (posts/tags/sets/etc) since you have a totally known DB state. It doesn't rely on previous tests passing/failing.