setup particle calculation on the gpu

Works so far, but on my (nvidia 650ti) it's slower than on the cpu..
Merge remote-tracking branch 'origin/master' into GpuParticles
GPU/CPU switch with "G" key press

fix cuda shutdown
Merge remote-tracking branch 'origin/master' into GpuParticles

Conflicts:
	src/particleemitter.cpp
	src/physicswrapper.cpp
	src/physicswrapper.h
	src/terrain/terraintile.cpp
	src/ui/eventhandler.cpp
Merge branch 'master' into GpuParticles
fix terrain changes while calculating particles on the gpu, add windows only ifdefs
add checks whether physx on the gpu / cuda is available when running on windows
Merge remote-tracking branch 'origin/master' into GpuParticles

Conflicts:
	src/physicswrapper.cpp
wrap cuda check in windows ifdef
Merge pull request #72 from lanice/GpuParticles

setup particle calculation on the gpu