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
Update README info on tests
Fix SQL error in Posts api when values array empty or not set

Need to pass at least 1 ID to NOT IN part of the SQL query.