Add rectangular aperture

This adds another aperture to the `aperture` subpackage - a Rectangular aperture.  This is useful for questions like "how much flux will be in my spectrograph's slit".

@kbarbary or @astrofrog should take a look at this, because I found the info on making a new aperture has some rather difficult to follow/missing documentation - in particular issues like sign (`x` appears to be the *second* index of the numpy array, consistent with e.g. fits files but not necessarily obvious).  That's not necessarily crucial, as probably the existing apertures pretty much cover it, but it just means I need to make sure I understand correctly what was meant here.

Also, I wasn't sure if I should add an `aperture_rectangular` function.  It would be easy enough, but of course one can also just give a `RectangularAperture` object to the `aperture_photometry` function.