Replaced SslClientStream with .NET SslStream

This change fixes an issue when connecting to a PostgreSQL database
configured to accept connections only with a valid client certificate.
With Mono's SslClientStream, an encryption error would occur when trying
to connect. WIth .NET's SslStream, the connection can be successfully
opened. The Mono-based callbacks have been marked "Obsolete". The newer
ValidateRemoteCertificateCallback is used automatically if it is
provided.
Merge pull request #27 from dreamlax/master

Replace Mono SslClientStream with .NET SslStream to fix SSL-related errors

Initial fix of the feature requests: 

[#1000445] Single npgsql.dll
http://pgfoundry.org/tracker/index.php?func=detail&aid=1000445&group_id=1000140&atid=593 

and

[#1005312] Remove dependency toward Mono.Security.dll
http://pgfoundry.org/tracker/index.php?func=detail&aid=1005312&group_id=1000140&atid=593