More DRY "My Account" menu

The "My Account" section menu items were repeated in the megamenu template
(``megamenu.html``) and the base template for the "My Account" section
(``storybase_user/account_base.html``)

I wanted a solution for rendering menus that:

* Allowed for identifying the currently active item
* Allowed registration of views from 3rd-party apps
* Could render the same menu with different surrounding HTML markup

I think I came up with a pretty DRY solution.  I added a new module
``storybase.menu`` that defines a ``MenuRegistry`` and ``Menu`` class.
A single ``MenuRegistry`` instance is instantiated and available as
``storybase.menu.registry``.

I create a ``Menu`` instance, add the url names corresponding to my views,
and register the menu in ``storybase_user.views``.

Finally, I created a new template tag library ``storybase_menu`` with
a template tag ``storybase_menu`` which renders a specified template with
a context that contains the menu items.

I broke out the markup for the two different instances of the menu into
separate template files: ``megamenu_account_menu.html`` and
``storybase_user/account_menu.html``.

I still need to document the ``storybase.menu`` module and the
``storybase_menu`` tag library.

Addresses #544.
Document menus feature

Document the classes and template tags used to implement the new
"My Account" menus.

Addresses #544.
Merge branch 'develop' into 544-my_acct_menu_order
Highlight the currently active item

Highlight the currently active item in the "My Account" sidebar menu.

This is achieved by giving the ``ul`` element the same ``sibiling-nav``
class as used for the Django CMS generated menus.

Closes #544