Added a dice library and changed the DnD 3.5 stat generator to use it.

I factored out the random bits that simulated dice rolls in the DnD 3.5 character generator into a dice utility library. It can be used to simulate dice throws by using the _dice notation_ (e.g. 3d6 to roll 3 dice with 6 faces). I also added some utility functions for summing the dice in a throw, interpreting a throw as a percentage, and discarding dice from throws.