Introduce 'export callback' to individual fields in export.inc. Add some documentation to export.html -- much of it borrowed from stella!

#596450 by Scott Reynolds: Provide an ajax command response to reset the modal to the loading state.

#589342: Introduce page_manager_get_current_page() to get information about the current page manager page.

Add cleanstring.inc tool to facilitate more pathauto-like functionality without necessarily using aliasing.

#545540 by Roger López: Add 5 page elements as available panes for using Panels as your primary page layout: breadcrumb, page title, messages, tabs and help box.