Implement bimatrix game generators

[tturocy]

At http://bimatrix-games.github.io/bimatrix-generators/ there are a number of interesting game types which make useful test cases, and also complement what exists in GAMUT.

It would be useful to integrate this with Gambit. The simplest solution would be to implement these in pure Python in, say, gambit.games.bimatrix. An alternative would be to incorporate them at the C++ level as well; perhaps that is something to be done in a further iteration.