Add abstraction for connection to "hide away" that whether the connection is TCP, or TLS.

Currently the way the data types are for rust-imap you must *know* the exact type of connection you will be using and which tls implementation you are going to use in order to define your types.

I would like to make a PR that abstracts that all away into an enum so that it is hidden from the user.

A user should be able to do something like this.

```rust
let mut builder = ClientBuilder::new(
self.config.imap_host.as_ref().unwrap(),
self.config.imap_port,
);
if use_starttls {
// tag that we want start tls
builder.starttls();
} else if use_ssl {
// tag that we are doing FULL ssl mode
builder.ssl();
}
// otherwise we are doing RAW tcp (still valid as it is useful for internal only connections)

// connects and returns an opaque ImapClient that abstracts away the Tcp stream type)
let client : ImapClient = builder.connect()?;
// connects and returns an opaque ImapSession that abstracts away the Tcp stream type)
let session : ImapSession = client.login("user", "pass")?;
```

Other crates do this and it greatly simplifies client-code as I don't have to know how I compiled the code in order to connect. And I can choose to conditionally connect via SSL / TLS / raw.

One example is ldap3, which uses a simple ENUM to differentiate between Tcp, Tls, and Unix sockets (imap would only need the 2).

Let me know if you are open to this and I can build up a PR.

No files or tests are named in the issue. Start by locating rust-imap's connection and client/session type definitions, then compare the existing TCP and TLS paths with the proposed enum abstraction; done means callers can choose raw TCP, STARTTLS, or SSL without knowing the concrete stream or TLS implementation types.

Written by the indexing model from the issue text.