WIP: Code coverage reports for base

A long time ago I noticed [a comment by simonster about code coverage in base](https://github.com/JuliaLang/julia/issues/7095#issuecomment-44986625) and finally took a stab at it.  Little did I know that I ended up with almost exactly [what timholy hacked together several moons ago](https://github.com/JuliaLang/julia/commit/15c9ef3b5a4ddcce91b64d2464b9de9ff9c1d517).

I put the coverage as a separate entry in the build matrix on Travis and explicitly allow for it to fail in order not to slow down the Travis reports.  As a consequence of this we will only run coverage on Linux, but I suspect that this does not matter.

However, there are several issues to consider before merging something like this (thus the WIP).

1. Do we really want Coveralls notifications for all pull requests?
2. Is the current quality of the coverage reports good enough?

For 1., if it is not possible to disable them on the Coveralls dashboard I can tuck the coverage code and Coveralls submission as optional make targets that can be run upon request rather than on Travis.  As for 2., I hope that someone else has more insights into exactly what the current shortcomings of the coverage code is.

Another note on the WIP status, I am yet to fire off the script onto Travis and may push to this branch shortly if I messed something up.