ENH: Stub for expectation value of 2-particle operator.

Added a SlaterDeterminantProduct class to deal with products of up and down
spin particles, and a get_expval method to calculate expectaton values,
given a particular potential.
ENH: Untested, complete version of two-body matrix elements.
TST: mbody -- Added tests for _get_orbmap.

We still need to find a way to test the actual matrix element
calculation.
BUG: Fixed bug in mbody.
ENH: mbody -- Added case for matrix element with same bra and ket.

Previously, when mbody.two_particle_matrix_element was called with
identical bra and ket (up to a permutation), it just returned the
expectation value. Now returns the correct answer.
TST: Added tests for mbody matrix elements.
DOC: Documented SlaterDeterminantProduct.
DOC: Documented two_particle_matrix_element.
DOC: Fixed private function docstrings in mbody.
Merge pull request #2 from scikit-quantum/slater_condon

Implementation of Slater-Condon rules for two-particle operators acting on Slater determinants.