Use up-to-date, Xamarin-friendly BouncyCastle submodule.
Update project file, silence warnings, etc.
Merge branch 'refs/heads/xamarin-studio'
Make producer info customizable.
Make it possible to use custom ISigner implementation.

It's a bit of a dirty hack, but oh well: change the PdfPKCS7 ctor so
that it doesn't require a private key and can take an initialized
ISigner instance instead.