Add a customizable sender email address.

Another attempt to resolve travis-ci/travis-ci#1048 and to get
distinct Gmail threads.
Remove brackets from the subject.

Circumstancial evidence from a helpful person on Twitter suggests
this is somehow ignored by Gmail, hence the grouping regardless of
sender and in-reply-to.
Add custom email headers for repository and result.

This allows more specific filters on the emails based on the
headers and e.g. the build results.

Fixes #1577.