Move quantity to a separate sub-package

This was @astrofrog and @mhvk's comments in #1577.  `Quantity` has grown into an extremely convinient and powerful tool for science.  But right now it lives in `units`, which both leaves it rather hidden in the documentation and makes it more difficult to work with.  So this is a suggestion to move quantity into its own separate sub-package.

Note that if we are going to do this, we should do it now (v0.3), because `Quantity` is useful enough that a fair number of users already have started with it between 0.3 and the next version.

This PR is not done as there are various parts of astropy that need to be adjusted for the new location (and docs moved), but it gives a flavor for this idea.  It shouldn't be too hard to finish it up quickly if we do decide to do this.

cc @mhvk @astrofrog @pllim @adrn @mdboom