Added keep-alive connection to SMTP. Fixed socket read/write timeouts. Added PHP useragent.

Email library now supports a 'smtp_keepalive' config, which will not create a new connection for each Email::send().

`$config['smtp_keepalive'] = TRUE;`

The timeout has been applied to the blocking read/writes as well.
`stream_set_timeout($this->_smtp_connect, $this->smtp_timeout);`

The useragent fallback is provided as PHP/version.