Changed travis script to run Python tests in parallel
Use a newer version of nose in travis script
Merge pull request #502 from peastman/travis

[WIP] experiments to figure out how to get nose running in parallel
Added a test case to make sure future changes to ForceField do not change forces
Merged upstream changes
Made test case more tolerant of numerical differences between platforms
Merge pull request #505 from peastman/forcefield

Added a test case to make sure future changes to ForceField do not change forces