Merge pull request #72 from peastman/master

__all__ should not include symbols that start with _
Merge pull request #74 from peastman/master

Changed order of arguments to produce a simpler API
Merge pull request #75 from peastman/master

Added SWM4-NDP water model
Merge pull request #76 from peastman/master

Updated to newer version of cl.hpp
Merge pull request #78 from peastman/master

Include process ID in temporary file names to avoid collisions
Merge pull request #79 from peastman/master

Added an argument to addHydrogens() for selecting which Platform to use for minimization. Also marked convertWater() as deprecated.
Merge pull request #80 from peastman/master

Lots of updates to documentation for new features in 5.2.
Fix segfault when positions are not 3 dimensional.
Merge pull request #82 from rmcgibbo/setLength

Fix segfault when positions are not 3 dimensional.
Cache the PTX files for compiled kernels
Cache filename specifies whether it was compiled in 32 or 64 bit mode
Eliminated some #defines to improve the efficiency of PTX caching
Can set OPENMM_CACHE_DIR to specify where PTX files should be cached
Merge pull request #84 from peastman/cache

Cache the PTX files for compiled kernels