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

Just a note on this - I think we should centralize both the error calculation function and the description of the error calculation since it will be in common between this code and the standard aperture photometry code.