Rename aux.go to common.go

The file name was causing problems on windows; see https://groups.google.com/forum/#!topic/gonum-dev/zyYSxiW9MVk and http://msdn.microsoft.com/en-us/library/windows/desktop/aa365247%28v=vs.85%29.aspx.
Merge pull request #19 from gonum/rename-blas

Rename aux.go to common.go