Add command line arguments, for item type specifiers, PEP-8 styling

I figured you might want to check this out. I made some of the variables easier to understand, used `enumerate` in places where indices were manually being incremented, and made a bunch of other changes enforced by PEP-8.

I also added a command line argument to change the output directory, and another to let users specify exactly which item types they'd like to download from a snapshot (can now choose between one or more of sms, voicemails, address_book, photos, call_history, and calendar). Check out the README for details.

Cheers!