Add close icon to alerts

Add a close icon that will remove an AlertView's element from the DOM.

Move the fade out functionality inside AlertView.

Add an onClose callback to the options of AlertView.

Set a cookie when the unsupported browser message is closed so the message
is not shown again when the user has closed it.

Addresses #548
Break alert management into a separate view

Alerts are now managed by AlertManagerView, instantiated by AppView instead
of AppView itself.
Better throttling of alert messages

Instead of only tracking the last alert messages, keep track of all visible
messages and only show a message if it isn't already visible.  This prevents
the stack of alerts that occurs when the layout is changed.

Also add a ``sticky`` option to the ``showAlert`` method that keeps an alert
message at the top of the list. This improves the behavior of the unsupported
browser message.

Addresses #548
Add dev section to changelog

This is a place to stage changelog messages for the current development
branch.  It avoids having to keep track of version numbers as hotfixes come
in.