Adds coverage report generation and serving

This takes the initial [coverage report script](https://github.com/basho/giddyup_coverage) and integrates it into giddy up proper.

Running make will generate an escript similar in purpose to the original script. However, the new script will upload results to s3 and relies on giddyup's internal sql query features and configuration idioms. This means that as giddyup changes, required changes to the script can be done in one pull request.

The script is intended to be run on the same machine that runs riak_test suites, allowing giddy up to maintain it's non-reliance on an actual riak install be accessible to it. However, since it does directly sql queries against the giddy up database, this may need to change.

This also adds another web machine resource to retrieve coverage data. It uses a similar strategy as the artifacts, simply piping them from s3 to the client.