Add velocity transformations to the coordinates package

The astropy.coordinates subpackage currently only supports positions (and some senses of time). A huge number of applications in Astronomy also require transformations between different velocity frames. E.g., observed velocity to heliocentric, heliocentric to galactic standard of rest, etc. There should be a pythonic framework for this that integrates well with the existing coordinates framework.

Note that this *might* include proper motions, but that could be addressed separately, depending on what users thing is a better-to-understand API. Probably the underlying machinery should be connected regardless, but it *might* make sense to have a user-facing API that hides some of those details.

Related GitHub issues: astropy/astropy#3093, astropy/astropy#3544, astropy/astropy#4344