notifier config improvements: db dir, email vars

- specify notifier_db_dir as a setting.  Had previously just stuck
  under $HOME, but Jim preferred as a distinct setting.

- pass through email settings env variables

- plumb through django's EMAIL_USE_TLS setting.  Required to be
  able to send mail using Amazon SES.  They speak smtp, but only
  over TLS.
Merge pull request #286 from edx/sef/fix-notifier-sqlite-location

Notifier config improvements: db dir, email vars