Summary Visualization

This is the completion of the summary visualization feature proposed and implemented by @jdalphon.

I modified the design of baseVis and baseHighVis to accommodate of these new types of vises, and well as changed the way histogram does field selection.  Furthermore, I added error checking to the summary panel to prevent the word null from creeping into the vis in the case where no field data was provided.

Please see my commit message for a more detailed explanation of what I actually did.

Testers
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- Play around with the new visualisation.  See if you can make the fields read 'null' (this would be bad).
- Play around with histogram. Also savedVis histograms should be verified as well.  Focus your efforts on the y-axis selection, and this is what was modified.
- Generally examine the visualizations to make sure none are broken. Double check that both current and new saved visualizations work as well.