Add error logging in Bugsnag (or equivalent)

[jipiboily]

TODO:

• [x] add decent logging all around (right now, the logs are empty if all is going well)
• [x] add error logging
• [ ] send errors to Bugsnag (in my case, make it configurable as much as possible)

[jipiboily]

Look at http://blog.bugsnag.com/go-errors & https://github.com/Sirupsen/logrus

[jipiboily]

First step was done with https://github.com/jipiboily/forwardlytics/pull/33