Created Unassigned: I have added support for SecureString [2614]

Hi there,
I would like to propose a change and add Support for SecureString type passwords. My proposal comes with a solution which you may use it if you like.

The change contains 3 new files:
SecurePasswordConnectionInfo.cs
SecurePasswordAuthenticationMethod.cs
SecurePasswordAuthenticationMethod.NET40.cs

and minor changes to:
NetConfClient.cs
ScpClient.cs
SftpClient.cs
SshClient.cs

where changes only add constructor for Passwords of type SecureString.

__There is an issue though, due to the fact that SSH is Encoding Passwords with Utf8 encoding, I was forced to make use of unsafe code in order to save SecureString as Unicode and then to Utf8 native-string.__