Added blivet_init.py snippet to easily initialize Blivet.

This script is for playing with Blivet interactively in ipython, e.g.:
$ ipython -i blivet_init.py

>>> sda = b.devicet<tab>.getDeviceByPath("/dev/sda")
>>> print sda
>>> sda.<tab><tab> # to see all method/properties
>>> sda.format.<tab><tab>  # to see all format methods/properties