Increases Document.slug max_length to 255

FileField paths have a limitation of 100 chars. With 255 chars for a slug we
can limit the filename by that, and still be able to cope with a *lot* of
repeated filenames.
Validates filename length

Fixes #37
Merge remote-tracking branch 'remotes/flavioamieiro/bugfix/validate-filename-length' into develop

Conflicts:
	pypln/web/static/css/pypln.css