Add Sprint.ly service for Crashlytics

This adds Sprint.ly integration to Crashlytics, building on the great work by @ablock in https://github.com/crashlytics/crashlytics-services/pull/17.

Add the integration in the app settings dialog:
![screen shot 2013-06-26 at 4 08 16 pm](https://f.cloud.github.com/assets/1056957/711890/3017cdae-de9c-11e2-8dfd-bec24fc0ed2a.png)
![screen shot 2013-06-26 at 4 08 29 pm](https://f.cloud.github.com/assets/1056957/711895/33ee7162-de9c-11e2-8ae6-bfa3902a5cbd.png)
![screen shot 2013-06-26 at 11 43 47 am](https://f.cloud.github.com/assets/1056957/711729/831f5c68-de99-11e2-8131-629a6fc85b81.png)

Receive an email about the new integration:
![screen shot 2013-06-26 at 4 07 40 pm](https://f.cloud.github.com/assets/1056957/711883/142bf0c0-de9c-11e2-98b0-f05d625e3ff3.png)

And when an issue reaches that impact level, the item is added in Sprint.ly:
(The counts are zero in the screenshot since this was artificially enqueued for testing).
![screen shot 2013-06-26 at 3 50 57 pm](https://f.cloud.github.com/assets/1056957/711742/c84326a8-de99-11e2-8c01-9f155c88fa4a.png)