Add some benchmarks for SkyCoord

@eteq This adds benchmarks for SkyCoord `repr` and `icrs` to `galactic` transformation, both of which I noticed are currently slow.

@mdboom I can't run the benchmarks on my machine, so this code is completely untested ... is it possible to run tests on pull request either on `travis-ci` or on `ci.scipy.org` or some other machine?