Import axpy! to BLAS and extend some BLAS 1 and 2 methods to work for StridedArrays

Right now the generic `axpy!` is called if imported from `Base.LinAlg`. With this change the BLAS version is called if possible. The extensions of the BLAS 1 and 2 methods are necessary for them to work for `ArrayView`s (and `SubArray`s).