Utility/example for deleting empty project versions older than <d> days

[OffBy0x01]

• Added new example for deleting empty project versions older than a given number of days
• Made datetime conversion utilities more robust
• Swapped own-implementations for builtin functions where appropriate

Was needing to write something like this anyway, though #173 gave me a little extra motivation :)

Note: I've not been able to fully test this script - please give it a go and let me know if everything looks right.

[skiyooka]

You may find https://github.com/blackducksoftware/sage/blob/master/sage_version_activity_to_csv.py interesting as it creates a .csv with one line per project version. The .csv includes all columns I can think of containing decision criteria for deletion. The intended pipeline is to run Sage, then the above script, then filter_activity.py to trim the list to the stuff one just wants to delete.