Interface changes

• HttpProxyClient class added.

• Proxy property added to FtpClient and FtpConnection class.

• Constructors with SSL paramaters added to FtpConnection class.

• CertificateReceived, CertificateRequired and WaitingForAsyncOperation added to FtpConnection class.

Behavior changes that may affect your projects

• None.

Other minor changes and bug fixes

• The State property now reflects the correct state after calling BeginChangeToParentFolder on an AsyncFtpClient object.
• When using the FtpClient or AsyncFtpClient class in PORT mode, listen sockets are now properly closed when an error code is received from the server.