Refactoring data API - Githubissues

Attempt to clean up the top level API.

Changes:

withData methods renamed and given documentation. New ones hopefully are easier to understand. Feedback welcome on better names though...
Top-level namespace was getting quite polluted. Pruned it a bit, and put some operations under objects (i.e. AggOps.Sum)
Added support for encoding values that are Dates (basically just formatted to the ISO-8601)
Added support for overriding how Any values are converted to JS friendly values, via an implicit argument.
Refactored various "type detecting code" into a vegas.data.* package. Added unit tests for this code.
Added unit tests for WindowRenderer, and tidied up some of that code (I didn't really understand what I was doing with JavaFX. Still don't really, but understand a little more I guess).

vegas-viz / Vegas