Readme update and test mocks

Hello Airbrake friends! 

Good News, we love Airbrake. We also love Go, we found this spiffy library earlier today, and happened across a couple small things we thought we could help out with. 

First:
We've updated the readme references to "Notifier.Panic" to "Notifier.PanicNotify" as it looks like there is no Panic function, and there is a PanicNotify that seems to do what we would expect. (Looks like this happened in the "Tweaks." commit, the last one on the log.)

Second:
We also noticed in doing this update that tests were expecting a local service to receive calls that would normally go to airbrake. So, we used net/http/httptest to setup a little fake server to receive and respond to those requests instead. (So peeps not working at airbrake can have passing tests too!) 

Thanks for a great tool and a Go library to implement it!