Fixing unit bug in aperture photometry

This should fix the unit bug @larrybradley reported in #122.

However temporary docs fixes in #139 will now brakes the docs build. I'm wondering which one is the desired approach:

 - Rebase the branch against master and add a commit that removes the #139 changes?
 - Rebase @astrofrog branch of #139 against this branch without the docs changes, and then the whole against master?