Custom weights

[mueller-fr]

For our use case it makes sense to use a customized weights matrix instead of the standard ones (non-weighted, linear, quadratic).

Changes

• Added custom weights, so the user can provide a weights matrix as a text file.
• Added command line option -w, --weighted to specify the file with the weights matrix.
• Added Math section to README with a description of the matrices.

Test Plan

• [x] check if the tests run with python2, still --> they do

Why custom weights?

We are dealing with sentiment ratings. Raters can vote for the regular "negative", "neutral", "positive" sentiments. But we also have something called "ambivalent" (when it is both positive and negative at the same time) and something like "data trash" (since the data comes from an automated system which does not always give readable text). This may be useful and sensible for someone else, so here is the PR :)