Add Commutator, AntiCommutator Operations

[goerz]

The operator algebra should include operations Commutator and AntiCommutator, although we probably don't want to automatically create them automatically from operator sums and products. Converting from an "expanded" to a "commutator" representation and vice versa is probably best done using the capabilities of #43 and #42.

The commutator and anticommutator routines in the superoperator algebra should return Commutator and AntiCommutator objects, unless expand is given as True.