Fixed isis2std bug where alpha channel was not being utilized for ARGB option....

Fixed isis2std bug where alpha channel was not being utilized for ARGB option. Fixed isis2std bug for TIFF and JP2 option that mapped Nulls to min output value. Modified ImageExporter and child classes to fit coding standards. Added user example and improved documentation to isis2std. Improved test coverage for isis2std and ImageExporter classes. Fixes #1380.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@5178 41f8697f-d340-4b68-9986-7bafba869bb8