Multiple changes.

- Remove the runtime dependency on rake-compiler.
 - Use #rb_str_new instead of #rb_str_new2, the hash character array
   isn't null terminated.
 - Add various safeguards for object instantiations.
 - Remove the 'threaded' options for messages, folding them into
   'archived'.  If archiving is enabled, so is threading.
 - Return nil for lookups from the list object instead of raising
   exceptions.
 - Open subject indexes with the proper encodings (thanks Michael
   Granger!)
 - Allow touching and unlinking files to operate on multiple paths
   at once, within a single safety() wrap.

spec/ezmlm/list/message_spec.rb

@@ -44,6 +44,7 @@ describe Ezmlm::List::Message do
 		it 'raises error when unable to read message' do
 			allow( list ).to receive( :listdir ).and_return( Pathname('/nope') )
 			expect( list ).to receive( :message_count ).and_return( 1 )
+			expect( list ).to receive( :archived? ).and_return( true )
 			expect {
 				described_class.new( list, 1 )
 			}.to raise_error( RuntimeError, /unable to determine message path/i )