Support new 12-6-4 potential

This potential was added to the Amber prmtop file in order to improve the computed thermodynamic properties for divalent metal ions.  For example, the 12-6 potential can reproduce _either_ the experimental hydration free energy _or_ the solvation structure (ion-oxygen distance and coordination number), but not both.  The 12-6-4 potential allegedly gets both right without resorting to a polarizable FF.  See the publication here: http://pubs.acs.org/doi/abs/10.1021/ct400751u

Tests for this potential are added.