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

SExtractor actually does something even more crude.  It interpolates masked pixels by simply copying the value of the previous pixel (usually the left-hand pixel), even if that pixel itself represents an interpolation.

There aren't any really good options to deal with bad pixels, but I think a mean of the immediate neighborhood isn't too bad.  One could even downweight the corner pixels (which are further in distance than the 4-connected pixels) in mean.  Please let me know if you have any better suggestions!  Ultimately, one can simply `exclude` masked pixels (the default) as well.

The mask interpolation is performed pixel-by-pixel, so I think the only case where *all* of the neighbors would also be masked is if there is a block of masked pixels around the origin pixel.