upgrade module to opencart 2.0.0

The changes reflect the opencart 2.0.0 standards.  There was an issue in
sucuresubmit.tpl that would not allow for '$this->config->get' so I was
forced to use 'global $config' and '$config->get' to allow the system to
fetch the test or public key.  All other development is correct.
final iteration of securesubmit module for opencart 2.0

Everything works as expected. You do need to have a SMTP connection(as I
found out the hard way) for the redirect to happen after form completion.
Merge pull request #4 from cmtracy/opencart_2.0_module

Opencart 2.0 module