Include EXEs and restrict composition pattern

FYI;

In the process of making [ScriptCs pack for SimpleWeb](https://github.com/ianbattersby/scriptcs-simpleweb) I made the following changes:

* Included EXEs as valid assembly references; probably an edge-case scenario but one I hit using Flux (Mark's little OWIN server) which is currently just a console app.

* Suspecting because SimpleWeb also uses MEF I found a conflict with the catalog composition in ScriptCs when Simple.Web DLLs were present in the bin directory, as such I've restricted the SearchPattern to "ScriptCs*.*". **I'm not confident this was wise** given there may be usage scenarios I'm unaware of.