Added async_copy() for vectors. Use like fast_copy().

Allow for overlapping host<->device communications with computations.
Added benchmark for host<->device copy performance.

Compares viennacl::copy(), viennacl::fast_copy(), viennacl::async_copy()
Merge branch 'master' of github.com:viennacl/viennacl-dev

Conflicts:
	examples/benchmarks/CMakeLists.txt