Mixed precision: Added conversion rountines for vectors and matrices.

Allows one to convert between {(u)int, (u)long, float, double} as needed.
Adds support for vectors and dense matrices (including proxies).
Support for viennacl::scalar<> already available via casts on host.
No support for sparse matrices for now, as no use case in sight.

Resolves #80.
Partially addresses #124: It is now easier to convert to the same types.