Remove references to gnutls_certificate_type_set_priority.

This function was deprecated at some point in the distant past, and has
now been removed from gnutls upstream. There is no good replacement, and
no current users of the Haskell wrapper, so the easiest solution is to
remove setPriority from the binding.

Fixups for AMP changes.

Remove unused FFI imports from gnutls-extra.

Bump version to 0.1.3.

Minor cleanups. Remove maximum version of "transformers" dependency.

Clean up the Cabal file, and move library source to lib/.