Fixed bugs in newly added CustomLennardJonesFluidMixed, which attempts to mix NonbondedForce and CustomNonbondedForce.
Modified CustomLennardJonesFluidMixture so that half of particles are CustomNonbondedForce and other half are NonbondedForce.
Added test to make sure system could be serialized and deserialized to produce the same energies and forces.
Merge remote-tracking branch 'upstream/master'
Merge pull request #129 from jchodera/master

Additional tests and bugfixes