add coverage badge once multiple packages can be tested

[stapelberg]

Commit 7b75a3c adds coveralls integration, see https://coveralls.io/github/robustirc/robustirc for the results. This however only runs tests in github.com/robustirc/robustirc, not github.com/robustirc/robustirc/...:

$ goveralls -package=github.com/robustirc/robustirc/... 
exit status 1: cannot use test profile flag with multiple packages

IIUC, this is because of a fundamental limitation of the cover command, see golang/go#6909

Once that issue is fixed, we should tell goveralls to run all our tests and then include the coverage badge. Until then, I don’t see much value in exposing a number which massively mis-represents our code/coverage.