Refactor the coalignment module

[wtbarnes]

The coalignment module now includes code from sunpy.image.coalignment as well as sunpy.physics.solar_rotation. Both of these bits of code provide methods for co-aligning images, but the API is a bit all over the place at the moment. We need to decide how we want users to interact with this code (e.g. do we want one mapsequence_coalign function with different keywords to select how the coalignment is done?) and refactor things to remove duplicate code.

We should also think about how we want to then incorporate new coalignment functions (e.g. #82).

This should probably be worked out prior to #79