Add SMTP support. Allows for email notification.

Email notification on:
   + Probe (recommended for LAN use only)
   + Successful login
   + User quit

Credit: https://github.com/jongreenall/kippo-dirtybastard/

-----

#### Probe email
Subject: `[Kippo] SSH Probe`
Message: 
```
There was an SSH probe request.
From: 127.0.0.1:52416.
To: 127.0.0.1:2222.
Kippo Session: 0.
```

#### Login email
Subject: `[Kippo] Successful Login`
Message: 
```
There was a successful login: (root/123456).
```


#### Quit email
Subject: `[Kippo] SSH Attack Finished`
Message: 
```
The attacker quit.

Please check the logs (log/tty/20140605-102239-552.log)!
```