Add code coverage + coveralls support

[springmeyer]

We should add coveralls support for per-commit tracking of C++ code coverage. The overall idea is:

• the node-osrm C++ code is built in debug mode and with the --coverage flag
• clang++ is used
• the job is run on OS X since it has a recent enough clang++ to support --coverage without issues
• the tests are run and the instrumented binaries dump metadata about what code paths where hit
• cpp-coveralls handles upload the data to coveralls.io (which needs the repo enabled)

node-fontnik is a good reference for how to do this: mapbox/node-fontnik#79.

[springmeyer]

first step done:

• enabled https://coveralls.io/r/Project-OSRM/node-osrm
• turned off the "leave comments" functionality since I find this annoying :)

Next steps: 1) enable coverage job + build settings, 2) start improving code coverage.