Sparkline: use add_note instead of add_menu

The Admin Bar is replaced with the Toolbar since WordPress Version 3.3. The preferred way to add items to the toolbar is with add_node().
See http://codex.wordpress.org/Class_Reference/WP_Admin_Bar/add_menu
Sparkline: display the sparkline in the dashboard as well

Fixes #621
Merge pull request #990 from jeherve/sparkline

Sparkline: use add_note instead of add_menu, and display the sparkline in the dashboard as well