adding check digit calculation

Hello,
I wanted to include the check digit calculation in the Vin class
is_valid() method. I also moved the WMI_info to the static.py file
where I added a few other things needed for the check digit calculation.

Now you could even remove verification.py, conversion.py, and wmi_map.py since they're all packaged in decoding.py and static.py

This is my first commit/pull on github so I didn't want to mess with removing anything.