Add Coveralls support

[smholloway]

Add Coveralls support for our Perl library.

[smholloway]

https://coveralls.io/r/toopher/toopher-perl

There are some issues displaying the coverage report because the coverage tool works against a directory that is not tracked by git, specifically blib -- the build destination. Locally, this works fine because you have the blib directory, but Coveralls and GitHub do not store it. I cannot find an easy way to point the output at lib instead of blib/lib so the report is correct but we cannot see the red/green coverage report through Coveralls.