Add pie chart for school view.

Add new pie.js in src/charts/
Add new utility functions in src/utils.js
Add to render function in SchoolView

To do: use CSS to color pie slices, instead of relying on the
order of keys in objects for consistency.
Make the templates look nice.
Watch out for SchoolView's render func getting too big.

Issue #48 Design and implement templates for individual metrics

by mheiber
Merge branch 'master' of github.com:mheiber/school-chooser