Publicly reexport TLS crates

cc #254

postgres/src/tls/native_tls.rs

@@ -1,5 +1,5 @@
 //! Native TLS support.
-extern crate native_tls;
+pub extern crate native_tls;
 
 use std::error::Error;
 use std::fmt;

postgres/src/tls/openssl.rs

@@ -1,5 +1,5 @@
 //! OpenSSL support.
-extern crate openssl;
+pub extern crate openssl;
 
 use std::error::Error;
 use std::fmt;

postgres/src/tls/schannel.rs

@@ -1,6 +1,6 @@
 //! SChannel support.
 
-extern crate schannel;
+pub extern crate schannel;
 
 use std::error::Error;
 use std::fmt;

postgres/src/tls/security_framework.rs

@@ -1,5 +1,5 @@
 //! Security Framework support.
-extern crate security_framework;
+pub extern crate security_framework;
 
 use self::security_framework::secure_transport::{SslStream, ClientBuilder};
 use tls::{Stream, TlsStream, TlsHandshake};