Add a Units tutorial

I think this is the first thing I want to teach folks here; it's super easy to use and incredibly useful on a daily basis.

I'd appreciate feedback - @kelle, a checklist for important things to add would be great.  Probably the starting place should be the astropy docs.  Right now the units demo isn't much different from the docs.  

@adrn, I think you had an equivalent notebook at some point?  Perhaps we should merge these?  Or just use yours?