adding extra checks when variable data is loaded to prevent crashing and some cosmetic changes to html report output

"glance info --parsable" now outputs the results in tab deliminated format, repeating the filename on each line, making it easier to parse programmatically. Additional fields may be added later!

At the moment only supports "info".  May add support for other formats later.

moving a huge pile of strings into constants modules and refactoring several groups of functions out of compare.py into different modules organized by their logical function

git-svn-id: https://svn.ssec.wisc.edu/repos/glance/trunk@201 8a9318a1-56ba-4d59-b755-99d26321be01