Feature/plotting

This PR is to move away from the stateful "implicit current plot" model of the current `bokeh.plotting` interface. 

* [x] `figure()` returns an actual `Plot` subclass, `Figure`
* [x] add glyph methods to `Figure`
* [x] add deprecations where appropriate 
* [x] verify existing examples work (but with deprecation warnings issued)
* [ ] update all examples
* [ ] update docs