Update displays.py

adds validation check to handle out of bounds values for _spdisplays_display-week
Update displays.py

corrections suggested by nbalonso
Update displays.py

Handle week=255 as indication of model year, not manufacturing date. See #138 and http://en.wikipedia.org/wiki/Extended_display_identification_data#EDID_1.3_data_format
Update displays.py

fixed typo
Update displays.py

typo
Update displays.py

eliminate complexity
Update displays.py

python grammar correction
Update displays.py

evidently the table doesn't like this, changing back to Manufactured
Update displays.py
@bochoven's suggestion
Merge branch 'pr/138' into wip