Add url mapping and view to retrieve subgenre data over time

Added an url mapping and a view to retrieve data on the percentages of
words that belong to some entity category for different subgenres over
time. The entity category is hardcoded to 'Posemo'. This should be made
dependent on the selected categories (a graph should be generated per
category). Also, a graph displaying the number of texts in the corpus
should be placed on top.
Show subgenres_stats_time bar graph in interface

Visualize the data generated by the subgenres_stats_time view on the
corpus partial.
Refactor data returned by the subgenres_stats_time view

The data returned by the subgenres_stats_time view used custom names for
the data points (year and percentage). This was changed to the more
generic names x and y, so the chart function can use a generic function
to find x and y coordinates. This prevents repeating functions to
extract the x and y values from the data.
Add utility function to extract parameters from the request body

Angular sends parameters in the request body. Added a module for
embodied_emotions utility functions. And added a function that
retrieves the value of a parameter given the key it was send under
(currently used to retrieve the selected entity categories).
Remove obsolete print statement
Fix url to get subgenres chart data over time

Added a missing /
Display subgenres over time graph for all selected categories

A subgenres over time graph is displayed for all selected categories.
For some reason, there is a large whitespace between the graphs. It is
unclear why this happens (the svg is larger than the values
supplied to the graph directive). This needs to be fixed.
Fix variable scale on y-axis

The y-axis is scaled based on the maximum y-value in the data. To allow
for easy comparison between graphs, the y-scale is fixed. Later the
y-axis will be scaled based on used defined parameters.
Fix whitespace below graphs

There was to much whitespace underneath the graphs. Removed the css
property that caused this from the svg element (changed from height:
100% to height: auto).
Merge branch 'feature/genre-percentages-over-time' into develop