Merge pull request #193 from peastman/master

Minor optimizations to CPU platform: avoid applying periodic boundary conditions unnecessarily
Merge pull request #195 from peastman/master

Skip all /internal directories when building Python wrappers.
include what you use
Merge pull request #200 from rmcgibbo/pgi

include what you use
Centralized the forces, positions, and thread pool so they can be shared between kernels
Bug fixes
Created optimized CPU version of GBSAOBCForce
Minor optimizations to CPU version of GBSAOBCForce
Merge pull request #205 from peastman/gb

Created optimized CPU version of GBSAOBCForce