Allow only single line messages in the subject.

Messages with newlines work with the default transports, but break mail headers in this one. {{msg}} in the subject could instead be thought of as a preview of the first line in the case of multi-line messages.

This is how ThunderBird displays a multi-line subject line.
![Capture](https://f.cloud.github.com/assets/48617/149991/e643ce72-7540-11e2-80c7-9708b2855dcd.PNG)