Initial commit of files from private dg repository to soon-to-be-public codap repository.
Configure SlickGrid and sproutcore submodules.
[From Bill] Graph scaling improvements; Rescale button in graph (IS-only)

Migrate Bill's latest changes from the dg repository to the new codap repository.

Preparation only - not yet fully implemented
* Double-clicking in the background of a plot zooms in. Shift-double-clicking zooms out. On first mousing over a graph, a hint about this shows for five seconds but then never shows again. (Not a long term solution. Just a trial balloon.)
Note: Alt-click and Alt-Shift-Click still work in this build but will go away soon.
Note: Double-tap on tablet not yet implemented.

strings.js [ tooltip text for double-click-zoom ]
data_tip.js [ Refactored so that tooltip functionality can be available more generally ]
plot_utilities.js [ Renamed constants for tooltips ]
dot_plot_view.js, scatter_plot_view.js, plot_view.js [ added handleBackgroundDblClick ]
plot_background_view.js [ Added a tooltip for the background ]
+ tool_tip.js [ Base class for refactored DG.DataTip ]

* InquirySpace only: A Rescale button appears in the top right of graphs that have at least one numeric axis. Clicking this button does the same thing as the Rescale to Data menu item (which will eventually probably go away).

strings.js [ tooltip text for rescale button ]
graph_view.js [ Added rescaleButton whose creation only occurs for InquirySpace ]
dg.css [ Styles for rescale button ]
graph_model.js [ Added hasNumericAxis and rescaleAxesFromData to support rescale button ]
+ rescale.png [ Image for rescale button ]
.gitignore to preserve lib folder and prevent inadvertently committing the autogenerated index.rhtml.