Upgrade packages only when notified.
Add manager method to return a ``Snippet`` by ``slug``.

https://github.com/madewithbytes/us_ignite/issues/172

Usage::

    Snippet.published.get_from_key('gigabit')
Populate the welcome email from an snippet.

https://github.com/madewithbytes/us_ignite/issues/172

The email content and subject are picked up from an
``Snippet`` entry. The slug of the snippet must be
``welcome-email``.
Load initial fixture for the welcome email.

https://github.com/madewithbytes/us_ignite/issues/172
Install ``sass`` and ``compass`` required by the front end.
Bugfix: Make sure ``grunt`` can access ``bower_components``.

Since the components are installed inside the assets directory
grunt needs to be told where it can import them.
Fix capitalization for ``Sign up`` in the navigation.
Update readme with testing details.