Adding coverage support for C code tests

Or, moving the goalposts for #722. 

Currently, we're at 78.85%, 7160 of 9081 lines covered.
With this patch, we're at 73.57%, 14400 of 19573 lines covered.

It installs a couple of extra packages, including, horror of horrors, a ruby gem. Using the extra bits, it combines the output json from gcov/lcov's c coverage with the json from coveralls-python. Then, the upload. 

The coveralls-merge package can be found in my repo: https://github.com/wiredfool/coveralls-merge