Add function to send welcome email to ``Users``

https://github.com/madewithbytes/us_ignite/issues/1

The function will send a welcome email to users
when they are registered or activate their account.

The function can be plugged directly to a signal.

Refers #1.
Workaround duplicated ``user_activate`` signal.

https://github.com/madewithbytes/us_ignite/issues/1

``django-registration`` by using the default backend
sends a duplicate signal ``user_activate`` signal.

Overiding the default ``activate`` method in the view
prevents the duplicate signal submission.

Refers #1.
Send welcome email when user activates account.

https://github.com/madewithbytes/us_ignite/issues/1

When a new user is created via Mozilla Persona, or
activated using the US Ignite registration mechanics
a welcome email is sent.

The welcome email is sent using: ``browserid.user_created`` signal
and ``registration.user_activated`` signals, because when
these signals are emitted the account is ready to be
used.

Closes #1.