Allow user to use aperture classes for the photometry

As discussed, this re-instates the use of aperture classes by the user (see also discussion in https://github.com/bsipocz/photutils/pull/1)

Here's an example usage:

```python
from astropy import units as u
from astropy.coordinates import SkyCoord
from astropy.io import fits

from photutils import aperture_photometry, SkyCircularAperture

hdu = fits.open('2MASS_k.fits.gz')[0]
gc = SkyCoord(0. * u.deg, 0 * u.deg, frame='galactic')

aperture = SkyCircularAperture(gc, r=5. * u.arcsec)
table = aperture_photometry(hdu, aperture)[0]
```

Just a couple of ideas for improving the suggestion in this PR:

* API-wise, I think the ``Sky...`` classes should also take a ``frame`` argument to specify what frame the e.g. position angle would be taken in (when relevant)

* We could allow pixel coordinates to be passed to ``Sky...`` (which would make ``frame`` default to ``pixel``) and we could then rename ``SkyCircularAperture`` to ``CircularAperture`` and ``CircularAperture`` to ``PixelCircularAperture``. The user then has a single point of entry (``CircularAperture``) and we preserve the simple ``PixelCircularAperture`` class as it currently is.

So example usage:

```python
gc = SkyCoord(0. * u.deg, 0 * u.deg, frame='galactic')
aperture = CircularAperture(gc, r=5. * u.arcsec)
table = aperture_photometry(hdu, aperture)[0]

pos = ([10,20,30] * u.pixel, [50, 60, 70] * u.pixel)
aperture = CircularAperture(pos, r=5)
table = aperture_photometry(hdu, aperture)[0]
```

I only implemented the ``SkyCircularAperture`` class for now, and will implement other ones and update the docs.

cc @cdeil @bsipocz 

These lines are not needed when running all the tests and actually cause doctest failure (it seemed strange to me as they are required when the test was run with ``-t`` for only this file, maybe it's a bug in py.test).