Fix setup.py unhelpfulness
Allow passing an allocator to ReductionKernel (patch by Simon Perkins)
Fix GPUArray.get{,_async}() docs. Fixes #58
Use appdirs package to create cache in XDG-correct location

This fixes #54.
Merge pull request #59 from bmerry/fix-54

Use appdirs package to create cache in XDG-correct location
Documentation config fixes