Add DataTables-based viewer for tabular data

This change implements a new viewer for tabular data (`data.frame` and friends). The viewer takes the place of the existing data viewer. It's built on the popular [DataTables](http://datatables.net/) library.

![image](https://cloud.githubusercontent.com/assets/470418/5309164/cd6c8244-7bd4-11e4-9dd3-2af5e162787a.png)

DataTables provides virtualized scrolling of large datasets, fixed headers with sort UI, and maintenance of view state, but all of the interesting data-related work (sorting, filtering, etc.) happens in R. The DataTables client bundles the client state and sends it to the server, which returns the slice of visible rows requested.

(Not ready for merge; needs review)
It's certainly the case that the data viewer windows open with a bunch of 304/Not Modified requests, but the time taken for those roundtrips is trivial compared to the cost of bootstrapping the data table (which is at any rate manually delayed until RStudio's finished animating the tab open).