Rework fixtures using PHPUnit DataSets

* Add PHPUnitFixtureContext to run fixtures
* Convert all manual fixtures to yaml
* Some minor changes to avoid different features tests clashing
Make "Updating post creates a new revision" test all 1 test

* Make Given... steps reset the restObject so we can make 2 requests
in 1 scenario
Speed up tests by only running fixtures per feature or as needed

* Added @resetFixture tag to trigger a fixture reset per-scenario
* Do general fixture run in before/after feature hooks
Fix case on datasets/ushahidi directory to fix tests
Merge pull request #58 from ushahidi/behat-phpunit-fixtures

Convert behat fixtures to use PHPUnit Datasets