Changes visualization url to accept the format
Accepts csv as an export format
Adds csv template for statistics visualization
Adds csv template for token frequency histogram visualization
Leaves the unicode escape to the template in the word cloud visualization
Adds csv template for word cloud visualization
Adds csv template for pos highlighter visualization
Adds document to the context dictionary right before rendering
Adds links to csv downloads in each visualization
Adds link to download the plain text extracted from the document

Fixes #23
Translate "download data" strings in visualizations
Merge branch 'feature/export-visualizations' into develop

Conflicts:
	pypln/web/apps/core/visualizations.py
	pypln/web/locale/pt_BR/LC_MESSAGES/django.mo
	pypln/web/locale/pt_BR/LC_MESSAGES/django.po
	pypln/web/templates/core/visualizations/token-frequency-histogram.html