Add Date header to emails.
Make package pass golint.
Simplify adding date header.
Fix typo in new comments.
Merge branch 'jcvernaleo-jcv_date'
Merge branch 'jcv_lint' of https://github.com/jcvernaleo/goemail into jcvernaleo-jcv_lint
Merge branch 'jcvernaleo-jcv_lint'