Add an electron unit

This is used heavily in ccdproc; having it in core astropy would be convenient.  Right now some CCD-related units are in astropy (e.g. ``u.adu``) but electron ends up being ``ccdproc.electron``. 

ccdproc already registers ``electron`` with ``u.add_enabled_units``,  so the only time this comes up  at all is creating a ``Quantity`` by doing something like ``q = 3 * ccdproc.electron``. The lengthier form ``q = u.Quantity(3, "electron")`` works.

This may well be something fixable on the ccdproc side -- if so, please let me know.