Segmentation photometry and morphological properties

This large PR adds two new functions to perform photometry and calculate morphological properties of sources defined by segmentation images (e.g, from SExtractor or made with `detect_sources`).  When the segmentation image is defined using a thresholded flux level (e.g., `detect_sources`), `segment_photometry` is equivalent to performing isophotal photometry in SExtractor.

I still need to write tests for these new functions, but this is ready for review.  I've done some comparisons to SExtractor, and with the same input segmentation image and background, the photometry and morphology results are identical.

Docs for updated `detect_sources`:
* http://www.stsci.edu/~lbradley/photutils/segmentation/api/photutils.detect_sources.html#photutils.detect_sources

Docs for new `segment_photometry` and `segment_properties` functions:
* http://www.stsci.edu/~lbradley/photutils/segmentation/api/photutils.segment_photometry.html#photutils.segment_photometry

* http://www.stsci.edu/~lbradley/photutils/segmentation/api/photutils.segment_properties.html#photutils.segment_properties

New `SegmentProperties` class:
* http://www.stsci.edu/~lbradley/photutils/segmentation/api/photutils.SegmentProperties.html#photutils.SegmentProperties

I just saw that there already is a central description of the error formulas used in photutils in the docs [here](http://photutils.readthedocs.org/en/latest/photutils/aperture.html#error-estimation). If this is correct, I don't see why the user couldn't compute `F` and `\Delta F` before calling any photometry functions!?

There could still be a utility function in photutils or ccdproc that computes `\Delta F` from the data and gain, but then all other functions (source detection, photometry) would not need this parameter any more, i.e. the API gets a bit simpler.

@larrybradley You mentioned that for source detection you really need separate `error` and `gain` parameters ... can you give a short code example where it's not possible to get the identical output from the source detection by first combining the inputs as shown [here](http://photutils.readthedocs.org/en/latest/photutils/aperture.html#error-estimation)?