Periodic boundary conditions and PME

Merge of pull requests #34 and #28 

Currently the force computation for electrostatics is missing.
Just a comment. This works only for boxes with orthogonal lattice vectors. Not sure if openmm supports anything else. 
All the best,
Andy
Sent from my iPhone

> On Nov 18, 2014, at 17:03, Andrea Zonca <notifications@github.com> wrote:
> 
> In platforms/reference/src/MBPolReferenceKernels.cpp:
> 
> > @@ -607,11 +611,13 @@ double ReferenceCalcMBPolDispersionForceKernel::execute(ContextImpl& context, bo
> >              throw OpenMMException("The periodic box size has decreased to less than twice the cutoff.");
> >          }
> >          dispersionForce.setPeriodicBox(box);
> > +        dispersionCorrection = dispersionCoefficient/(box[0]*box[1]*box[2]);
> @gmedders it is divided by the volume down here.
> 
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