Fixed #58.

Initial attempt to fix this issue,

1. When assemblies are being registered, find PCL ones and add their profile paths to search paths.
2. Suppress `CopySecurityDeclarations` exceptions as PCL ones do not have security declarations.

Both changes are experimental as I only run through initial testing on simple cases.
@gluck The one I used is also .NET 3.5 compliant. If you prefer the SO version, I can later submit another revision to address that.