Change snapshot-dir.py to snapshot_dir.py and extend

[bartlettroscoe]

This story is to address several issues with snapshot-dir.py:

• Change name from snapshot-dir.py to snapshot_dir.py and make a stand-alone script: By only using underscores _ and not dashes - in the name, this will allow snapshot_dir.py to be a python module (so we can do away with SnapshotDir.py) and then have just have the unit test module snaphshot_dir_UnitTests.py. This will allow people to copy out this one stand-alone script and use it as part of their workflow outside of TriBITS.
• Extend snapshot_dir.py to snapshot from Hg and SVN repos into git: The only hard part here will be asserting the origin Hg or SVN repos are "clean" and to extract the version info from the Hg or SVN repos needed to build the git commit to provide perfect backtracing. This will allow this script to be used to integrate repos from Hg and SVN into git projects. This will be used, for example, to implement the syncing of MOOSE (SVN) and SCALE (Hg) into git copies for VERA.
• Skip final commit if there are no changes to commit: This will allow this to be used in automated scripts and workflows without failing if there are no changes.
• Add --exclude arguments to allow additional directories to be excluded as well: This will be needed to implement snapshot_tribits.py (see #26).