Make Npgsql use SSLStream for ssl connections instead of using Mono.Security.

Mono.Security assembly is used by Npgsql to provide ssl support.
SSLStream class found in .Net class library also provides support for ssl.
Before this patch, Npgsql used Mono.Security by default.
With this change, it will use SSLStream class found in .Net class library.
Merge pull request #275 from franciscojunior/change-default-to-SSLStream

Make Npgsql use SSLStream for ssl connections instead of using Mono.Security