moved sn validation to validators & added max_length=200 check on single sn
skipping empty sn & fixed test message
MultilineField used on sn in add part|device
rewritten sn, barcode, imei in 'add device|part' form to new MultilineField
Merge branch 'develop' of https://github.com/allegro/ralph_assets into unlimited-multi
better docstring in validate_snbcs
Merge pull request #196 from xliiv/unlimited-multi

Added MultilineField.