Implements Debug for `TlsConnector`

[sgg]

Closes #99

Derived for Windows and macOS, hand-rolled for Linux due to a missing downstream Debug dependency. I whipped up a little test script and ran it across Linux, macOS, and Windows and to my eyes it doesn't appear to be leaking anything sensitive.

---

extern crate native_tls;

use native_tls::{Identity, TlsConnector};
use std::fs::File;
use std::io::Read;
use std::path::Path;

fn main() {
    let pkcs12 = File::open(
        Path::new(env!("CARGO_MANIFEST_DIR"))
            .join("test")
            .join("identity.p12"),
    )
    .map(|mut f| {
        let mut pkcs12 = vec![];
        f.read_to_end(&mut pkcs12).unwrap();
        pkcs12
    })
    .unwrap();

    let tls = TlsConnector::builder()
        .identity(Identity::from_pkcs12(&pkcs12, "mypass").unwrap())
        .build()
        .unwrap();
    println!("{:#?}", tls);
}

Linux

```rust TlsConnector( TlsConnector { use_sni: true, accept_invalid_hostnames: false, accept_invalid_certs: false, }, ) ```

MacOS

```rust TlsConnector( TlsConnector { identity: Some( Identity { identity: SecIdentity { certificate: SecCertificate { subject: "foobar.com", }, private_key: SecKey, }, chain: [ SecCertificate { subject: "Internet Widgits Pty Ltd", }, ], }, ), min_protocol: Some( Tlsv10, ), max_protocol: None, roots: [], use_sni: true, danger_accept_invalid_hostnames: false, danger_accept_invalid_certs: false, disable_built_in_roots: false, }, ) ```

Windows

```rust TlsConnector( TlsConnector { cert: Some( CertContext( 0x000002205c2638c0, ), ), roots: CertStore, min_protocol: Some( Tlsv10, ), max_protocol: None, use_sni: true, accept_invalid_hostnames: false, accept_invalid_certs: false, disable_built_in_roots: false, }, ) ```