ENH: Addition of IntensitySegmenter as a Slicer extension

IntensitySegmenter is a simple tool that segments an image according to intensity value. It is mainly used to segment CT scans using the Hounsfield scale but the ranges of intensities and their corresponding labels can be specified in an input text file. For more information, refer to [1] and [2]

[1] http://www.slicer.org/slicerWiki/index.php/Documentation/Nightly/Extensions/IntensitySegmenter
[2] http://www.nitrc.org/projects/dentaltools/