Change blas_module to CLBlast instead of CLBLAS?

[ranocha]

It might be worth discussing to change the BLAS backend for CLArrays to CLBlast.jl instead of CLBLAS.jl. CLBlast is "a tuned OpenCL BLAS library". While it is not necessarily better in all cases than CLBLAS, the GEMM routine has been optimised a lot. Moreover, CLBlast seems to be more actively developed. If there is some consensus in changing the blas_module, I can implement additional highlevel wrappers such as dot(x::CLArray).

[ranocha]

cc @SimonDanisch