XML documents can now be signed and can check their signatures
Specify key for XML signing
comet-sendvo can sign voevents
Simple signature validator
Hash events based on contents only

Rather than potentially changeable signatures.
Update DummyEvent; fixes failing tests
Fix failing tests
Tests for GPG signatures
Remove unused import
Add check_auth decorator
Add IAuthenticable
check_auth checks for IAuthenticatable
implements() -> @implementer for zope.interface 4.0.0
Broadcaster only sends to authorized subscribers
Add stub authenticate method
Test VOEventBroadcaster interface
Only test authentication if enabled
Split authenticate method into mixin class
Broadcaster actually checks sig for auth
Factor GPG test support code into reusable superclass