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.

So, we lose percentage, but gain instrumentation of 10k lines of code. 

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

Output can be found here: https://coveralls.io/builds/1029743 , especially this file:  https://coveralls.io/files/257210494

Performance ranges from not bad to a somewhat worse than just python coverage. We may want to consider covering 2.7+sitepackages, 3.4, and maybe one of the pypys, as that should get us accurate to within a few lines of coverage. (edit, according to my tests, those three (2.7+site, 3.4, pypy2.3) gets us +- 1 line of coverage compared to all 8 of them.)