Implementation of CA, RDA and CCA.

Broadly, the main structure is a super class that know how to plot biplots
(ordination results), and then subclasses that implement an
scores method that computes a standard set of results.
TST: Add testing files

There're still some more tests to be added, in particular for RDA
and CCA.
Make package installable (add setup.py)
DOC: Update readme