Exports raw freqdist data in the csv for Token Frequency Histogram visualization

Fixes #27
Fixes position of document upload form submit button

Fixes #34
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 pull request #35 from flavioamieiro/bugfix/fix-token-histogram-visualization

Exports raw freqdist data in the csv for Token Frequency Histogram visualization
Merge pull request #36 from flavioamieiro/bugfix/upload-button-position

Fixes position of document upload form submit button
Merge remote-tracking branch 'remotes/flavioamieiro/bugfix/validate-filename-length' into develop

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