Teamcity coverage support

The teamcity logic for combining and reporting coverage is moved to ``tools/teamcity-coverage-report.sh``

The final teamcity build step simply runs 'coverage combine' and 'coveralls' to report coverage to coveralls.io.  When viewing the build results, one can now see the coverage change change introduced:

https://teamcity-master.pexpect.org/viewLog.html?buildId=853&tab=buildResultsDiv&buildTypeId=Pexpect_BuildHead

I'm also enabling TeamCity's github status.  We should see it in this PR? I'm expecting that *both* travis and teamcity may contribute to a single status, and they should both be using a unique "context" key.