Impl/error source

[eldesh]

I added source method of std::error::Error for Error and imp::openssl::Error.
This PR enable users downcast_ref errors like:

let tls_error: native_tls::Error = ...
if let Some(ref ssl_error) =tls_error
    .source()
    .and_then(|e| e.downcast_ref::<openssl::ssl::Error>())
{
    ...
}

[sfackler]

You can just remove the cause implementations - they'll delegate to source anyway.

[sfackler]

You should be aware that we don't consider the underlying TLS implementations to be public API. We can and will e.g. upgrade versions without a breaking release.

[eldesh]

It is advisable to provide precise error type from this crate.
But acquiring inner error type using source method is reasonable.
Also, the method is required for standard error trait.
A user code depends on an inner error type, breaking can't be avoided.