Switch to jsonlite

This might be a breaking change to the existing widget packages, but I think it should be easy for them to adapt to this change. My personal experience is that, among several other advantages, jsonlite covers the corner cases much better than RJSONIO. The default treatment of data frames also makes better sense to JS libraries (record/row-based instead of column-based). Another bonus point is @jeroenooms is extremely responsive, so we can bug him before he becomes a busy super star :-p

I removed the `to_json()` function because 1) `jsonlite::toJSON()` already does the "right" thing; 2) Joe has provided a handy helper function `HTMLWidgets.transposeArray2D()`. I do not see the need for `to_json()` any more.