Scheduler: Removed unused execute_matrix.hpp
Scheduler: Removing unused execute_vector_assign.hpp
Code Cleanliness: Header files are no self-sufficient, i.e. can be included standalone.

Two exceptions:
 - viennacl/generator: map_functor.hpp leads to a circular dependency, so cannot (and should not) be included directly.
 - viennacl/rand:      Currently not in use, prepared for inclusion in a later release.

Fixes #48