Add notifications support.

* src/cuirass/mail.scm: New file.
* src/cuirass/notification.scm: New file.
* src/sql/upgrade-5.sql: New file.
* Makefile.am (dist_pkgmodule_DATA, dist_sql_DATA): Add them.
* src/schema.sql (Specifications)[notifications]: New field.
* bin/cuirass.in: Wrap inside the "with-notification" macro.
* src/cuirass/remote-server.scm (remote-server): Ditto.
* src/cuirass/database.scm (db-add-specification): Add notifications field.
(db-get-specifications): Ditto.
(db-update-build-status!): Send notifications.
* tests/mail.sh: New file.
* tests/database.scm (mu-debug, tmp-mail): New variables.
(mailer): New procedure.
(example-spec): Add an email notification.
("mail-notification"): New test case.