Add tests

I added some tests for `HOOHelper`, `HOOErrorGenerator` and `HOOHoodie`. See https://github.com/kaalita/Hoodie-iOS/issues/11.

PS: I also changed the [travis.yml](https://github.com/pietbrauer/Hoodie-iOS/commit/5b3ace1c0f52c1774f969f366f901a7c5408807e) to use xcodebuild instead of xctool, because in some version xctool is unreliable and [xcpretty](https://github.com/supermarin/xcpretty) has [better build output](https://travis-ci.org/kaalita/Hoodie-iOS/builds/25394918).

PPS: There is another PR coming for some refactoring in `HOOAccount` also with tests.

PPPS: Maybe you should consider removing the Pods directory from the project as the [Cons](http://guides.cocoapods.org/using/using-cocoapods.html#cons) are not really applicable for this project. Also it makes PRs a bit shorter (I didn't actually changed 202 files).

Have a nice weekend.