Move view tests to their own package
Separates pos highlight from other visualizations (with minimal tests)

This is the only visualization that needs pagination, so it should be
different from the others. This commit also adds a minimal set of tests. The
idea is to simplify this view a lot, since it won't have to deal with the
different kinds of visualizations. For now, the view was only copied.
Cleans up pos_highlighter_visualization view a bit
Adds test for pos highlight csv export
Turns pos_highlighter visualiation into a class based view

The idea is to isolate the logic as much as possible, and remove it from
VISUALIZATIONS. We can do this by having a base class that has all the common
behaviour and letting child classes implement what is specific (like what's in
the 'process' key in VISUALIZATIONS[visualization]).
Creates a base VisualizationView class
Makes apps.core.views a module
Moves the new visualization views to their own file
Makes 'render_to_response' generic and moves it to the base class
Uses the visualization slug to find the template
Adds label to the visualization view

With this, all the information that was in the VISUALIZATIONS dictionary is now
in each VisualizationView subclass.
Merge remote-tracking branch 'namd/develop' into feature/refactor-visualizations

Conflicts:
	pypln/web/apps/core/views/__init__.py

I needed the code that changed in the part-of-speech visualization
Adds test to verify if an e-mail is sent in case an unknown tag is found
Removes all code related to POS from the old visualizations module

Now the Part-of-speech visualization uses only the new infrastructure. Now we
can start porting the other visualizations.
Normalizes POS visualization as 'Part of speech'

There should be no more references to pos-highlighter
Removes test format from url
Makes returning data from store the default behaviour for `process'
If the required format is 'txt', returns a content type of 'text/plain'
Migrates 'Plain text' visualization to the new structure

This also adds minimal tests for the view
Migrates 'Token Frequency Histogram' visualization to the new structure

Adding some tests