Added encryption feature to svn-backup-dumps.py tool

hi

I have extended the options of svn-backup-dumps tool which now enables
creating encrypted dumps (using AES256-CTR).

It has a dependency on python's pycrypto module (which is by far the most
popular crypto module).

If you want some explanation about the encryption scheme then drop me a
mail although I think the python code and the bash script are
self-explanatory.

Ben