Glicko2 initial release

This library implements the Glicko2 algorithm. It is composed of 3 modules to be used independently, depending on your needs:

• A low level module which allows to rate multiple games within a rating period, the exact algorithm described in the paper.
• A single game convenience module which is easy to use and allows you to rate games one by one, as they are played, and get new ratings for the players involved.
• A functor which allows you to specify low level variables from the Glicko2 paper, and some other properties.