Add an airbrake hook in a package.

Code taken from https://github.com/Sirupsen/logrus-hooks/blob/master/airbrake/airbrake.go with some changes to avoid users needing to import `"github.com/tobi/airbrake-go"` to set global vars in it.

Also add example in docs, replace occurrences of `exception` for `error`, except in the sentence:

> an exception-tracking service compatible with the Airbrake API

r:@Sirupsen, @snormore, @burke