Add a semi-unique In-Reply-To header to the email.

Gmail tends to group messages for the same build into a single
thread, making it confusing when the build first had an intermitten
error and a subsequent build has passed. This should give Gmail
more unique features on which to distinguish an email thread.

Relevant to travis-ci/travis-ci#1048
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.