Add Coveralls

Coveralls is a web service to help you track your code coverage over
time, and ensure that all your new code is fully covered.

More info:

  https://coveralls.io/docs

Change-Id: I1bbda3a5796486c570f508afd41ddf8ed96a11f8
Reviewed-on: http://gerrit.causes.com/35947
Reviewed-by: Henric Trotzig <henric.trotzig@causes.com>
Tested-by: Joe Lencioni <joe.lencioni@causes.com>