Add coveralls support and fix a few testing bugs related to coverage

This PR adds coveralls support (https://coveralls.io/) to ZF2. This tool integrates nicely with both GitHub and Travis, and allows us to keep track of our code coverage.

In the same run I fixed a few broken ```@covers``` annotations and a bug which prevented complete testing on Ubuntu 12.04 (see comments in the diff).

Coveralls status can be found at https://coveralls.io/r/zendframework/zf2