ssl: use std::ptr::Unique to fix for opt-in Sync

Modifies Ssl & SslContext (there are others I haven't modified because I didn't need them for r2d2-postgress to build).

This removes the errors like:

	src/ssl/tests.rs:202:14: 202:27 error: the trait `core::kinds::Send` is not implemented for the type `*mut libc::types::common::c95::c_void`
	src/ssl/tests.rs:202     let _t = Thread::spawn(move || {
                                          ^~~~~~~~~~~~~
	src/ssl/tests.rs:202:14: 202:27 note: the type `*mut libc::types::common::c95::c_void` must implement `core::kinds::Send` because it appears within the type `ssl::Ssl`

We may want some locking around calls that do multiple operations on
the underlying Ssl and SslContext objects, but this lets us preserve
functionality for now.