Dashboard layout modifications

Upgrade jquery to 1.9.1
Add jquery.shapeshift plugin to layout dashboard widgets. http://mcpants.github.com/jquery.shapeshift/
Add rockstor_widgets.js, a registry to available widgets, their views, and default configuration options.
Modify dashboard configuration model to load and save a json list of widget names, positions and sizes.
Change renderWidgets in home.js to
- load saved dashboard  configuration if it exists, or use the default one from rockstor_widgets. 
- render widgets in a list and use the shapeshift plugin for layout

Add addWIdget and removeWidget methods in home.js to enable add and removal of widgets.
Modify DashboardConfig view to trigger a destroy of shapeshift, add / remove the selected widgets, and call shapeshift again so that rendering the new widgets happens properly (https://github.com/McPants/jquery.shapeshift/issues/34)
Add jquery.flot plugin for grpahs.
Modify the rendering methods in cpu_usage_widget to draw stacked graph using the flot plugin.
Modify the cpu widget template and style so that the width and height of the graph placeholder can be set as a percentage, so that the flot resize plugin is able to automatically resize the graph.