Scheduler: Added support for A = trans(B)
Matrix-Vector-Products: Moved operator+= from vector_base to viennacl namespace.

This fixes compilation problems on Visual Studio, which (wrongly?) fails
to resolve an ambiguity correctly. The reasons seem to be that in-class
implementations get higher priority over operator overloads implemented as
free functions.
ChangeLog: Added changes for 1.5.2 release
Updated version number to 1.5.2.