Support for integer types

[karlrupp]

We should support viennacl::vector and the like.
Pitfalls:

• Operations on vectors with different type, e.g. adding vector to vector
• Result type of norm_2(x) for an integer-vector?
• Conversion between host types and OpenCL types (cl_int and int might have different binary format). Fortunately this is already addressed with the new viennacl::backend::typesafe_host_array<> implementation.

[karlrupp]

Operations on vectors with the same type now supported. Unsigned integer vectors do not support subtractions so far. Closing issue now, please open more specific issues as needed.