Mail notifier

I have refactored the Notifier logic a bit.

They are now loaded dynamically, such that the pony / mail / log4r gems do not need to be explicit dependencies.

I have added a MailNotifier that uses the `mail` gem directly, bypassing the `pony`.

These changes are 100% API compatible, with the only end-user difference being that `Settings#outputs` no longer defaults to `[:io, :email]`.  It must default to `[]` in order to have no default dependencies.  This allows to gem to load in any environment without the other gems installed (log4r, mail, pony).

I have added support for automatic conversion for the old output identifier `:email` to `:pony` with a deprecation warning.

I have added specs to cover the new MailNotifier.

This pull request includes the commits from my other PR #5, so you can close that one, or merge it first.