ENH: Split segmentation algorithm code from the application

I put all the segmentation algorithm code in a separate file,
AirwaySegmenter.hxx, so it can be called from other applications.
ENH: Separate command line parsing from the actual algorithm code
ENH: Put algorithm into function that takes an input and output image

This separates the algorithm from reading and writing of files by the
standalone program.
STYLE: Removed trailing whitespace
DOC: Fixed a comment
STYLE: Cleaned up function signatures and added some comments