Replacement of getexecutingassembly

In Windows Store getexecutingassembly is not exposed, since it is not
guranteed to give proper answers e.g. in method inlining - hence deriving
assembly from a well-known type.

At this point all code compiles in both targetted profiles. If I understand the docs correctly at http://docs.nuget.org/docs/release-notes/nuget-2.1, the following directory names should apply for the compiled dlls

* Debug  -> portable-win+net40
* NET45WP8 -> portable-windows8+net45+wp8