Adds coveralls.io support

This PR is a followup to #256, JaCoCo coverages will be generated by travis-ci and published to coveralls.io, the report will be here: https://coveralls.io/r/bitcoinj/bitcoinj

This is required by someone with github access:
1. Signup on coveralls.io
2. Add repository "bitcoinj"
After travis builds a project you should see code coverage reports there and also the badge on README.md page will show correct values.