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 don't have a better suggestion how to interpolate masked pixels.

Is it possible to make this a separate function (e.g. called `interpolate_masked_data`) that the user calls before calling any of the source detection / photometry routines?
If yes, I think it's worth doing as part of the public API?
If no, maybe it could still be shared code by different functions in photutils as a private utility function?