Add testMode to SlmMail

I've started work on adding a test_mode config value.

When test_mode is true, SlmMail wont actually send any emails.
This is needed when doing QA or testing. You might want to test with real costumer data, but not actually sending emails.

Right now when test_mode is true i just return an array instead of doing the actual $response->send. I don't know if this is the right way to do it?

Will add unit-test when i've gotten clarified if `return array()` is correct.