Add GitHub issues integration

This builds off the initial work from @linjunpop in https://github.com/crashlytics/crashlytics-services/pull/24 and adds GitHub issues integration to Crashlytics!  It uses personal account tokens, which developers can control through the GitHub UI, and (see screenshots below), our recommendation is that users create a specific GitHub user for this integration with limited access.

This also updates the screenshot on the README.

Adding integration:
![screen shot 2013-08-30 at 1 44 37 pm](https://f.cloud.github.com/assets/1056957/1059566/a2ec727e-119d-11e3-93e6-c2a4dbe57f10.png)

![screen shot 2013-08-30 at 1 44 11 pm](https://f.cloud.github.com/assets/1056957/1059569/a7446462-119d-11e3-8031-9c2081f159dd.png)

GitHub issue:
![screen shot 2013-08-30 at 1 59 03 pm](https://f.cloud.github.com/assets/1056957/1059583/df3ce3a8-119d-11e3-8fd6-6e831e633ee5.png)