Iterative: Added convenience overload for STL types.

A library user can now call CG, BiCGStab, and GMRES directly
with STL types (vector<map<T, U> >, vector<U>).
Internally calculations are forwarded to the corresponding ViennaCL types.
Requires the user to include compressed_matrix.hpp and vector.hpp before
the respective solver implementation. This is not great, but the
best we can reasonably provide at this point.

Resolves #12.