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.
I am going to report that to Cecil and put back an issue reference. Cecil fails to return `false` even for `meth.HasSecurityDeclarations` if the method comes from a PCL, but throws an exception where this `try-catch` aims to eat.

I don't like catch all, but this exception should not be handled by IL-Repack either. Hope Cecil guys can fix that on their side.