Reworked kernels for vectors: Now part of the source tree, no longer generated via conversion scripts.

Now all element-wise kernels other than addition and subtraction are managed by viennacl::linalg::opencl::kernels::vector_element.
Remaining kernels are managed by viennacl::linalg::opencl::kernels::vector.

The reason for this change is that this gives us more flexibility with respect to
providing device-specific kernels later. Also, it is one step in making the developer
repository more accessible to new users, particularly Windows user.