Adding YouTrack service integration, refactored and with tests

This builds on the awesome work from @gilles in https://github.com/crashlytics/crashlytics-services/pull/30 to add YouTrack integration to Crashlytics!

Adding integration:
![screen shot 2014-01-14 at 12 15 51 pm](https://f.cloud.github.com/assets/1056957/1912792/8aeeb0ac-7d3f-11e3-86de-53a57e1d7dc1.png)

![screen shot 2014-01-14 at 12 16 00 pm](https://f.cloud.github.com/assets/1056957/1912794/8c59a424-7d3f-11e3-97fc-a52f24952118.png)


New integration email notification:
![youtrack](https://f.cloud.github.com/assets/1056957/1912804/a5905e10-7d3f-11e3-9593-65bcdb415c41.png)

YouTrack issue:
![youtrack issue](https://f.cloud.github.com/assets/1056957/1912811/ad62913a-7d3f-11e3-8371-89c39ddb0770.png)