`pipeTo`: Forward write errors to the reader.
Reintroduce data for proprietary pings.
Add coverage reports.
Enable Coveralls integration.
Test on Go 1.4.
Retry GCM requests.
Merge pull request #149 from mozilla-services/feature/propping-data

Reintroduce data for proprietary pings
Merge pull request #150 from mozilla-services/feature/cover

Add code coverage reports
Remove the duplicate client check. Closes #141.