Make goblas the default BLAS implementation

Also allow cblas use in testing with `go test -tags cblas`.
If you want to test with cblas, use `go test -tags cblas`. This does not happen on travis. If you do this locally, it will fail at the moment - we need to harmonise panic values between the two implementations. My proposal would be to drop the c from the beginning of the cblas panics and use those in both - the reasoning being that those messages are more informative (they give a stringer hint to which parameter was incorrect).