Update version number for release 13.1.2.
Merge branch 'maint13' after release 13.1.2.
Remove GSoC 2014 advert from website front page.
Implement normalize operation on mixed profiles.

Mixed strategy and behavior profiles have never been required to sum to
one, as the entries in the vectors can usefully be interpreted as, for
example, counts rather than probabilities.  However, normalization of
a profile such that each player (or information) set's entries summed to
one, to be a proper probability distribution, has never been implemented.

This commit implements a Normalize() (in C++) and normalize() (in Python)
operation, which does just this. For any player or information sets where
the sum of entries is non-zero, the entries are re-scaled to sum to one.
In the case the sum is zero, the entries are left at zero.
Refactor strategic form LP solver into Nash equilibrium class framework.
Refactor sequence from LP solver into Nash equilibrium class framework.
Remove old implementation of subgame solver (superseded by new Nash framework).
Merge remote-tracking branch 'upstream/master'