Update to 4.6 (global_context) all tests **passed** and all examples are compiled with success on Windows 8.1 (VC 2012)

On 0.4.6 Boost.Singularity is not needed anymore.
Some fixes on:

- ensure_single_instance.hpp
- launch.hpp
- ensure_single_instance_test.cpp

Now we are working on some improvements on interface.

Thanks to Benjamin Dieckmann for global_context implementation!