ProtoSocket

Added SSL example, bug fixes and upgrade refactor

- Fixed a bug where the PeerConfiguration constructor ignored the protocolMode argument and always set the mode to active
- Changed the way UpgradeAsync works, it now requires that the peer be in Passive mode, simplifying the logic but potentially a breaking change
- Added SslUpgrader.CheckCertificateRevocation
- Added SslUpgrader.RemoteValidationCallback
- Added SslUpgrader.LocalSelectionCallback
- Added an SSL text-protocol example
- Moved to 0.6.0