Added some tests and fixed parsing to support the way monit sends email

Hi,


we are working on capturing emails from monit to redirect the alerts using our go program.
When trying it we ran into the following error:

```
invalid MAIL arg: "FROM: <monit@72511f54-196e-4cdc-aeab-54ad70db783d>"
```

So we added a couple tests to cover the basic sending of an email with or without monit style headers...


Thanks!

Signed-off-by: Ben Smith <bsmith@pivotallabs.com>