Handle document creation directly

[philippjfr]

In the latest bokeh dev builds the _ModelInDocument context manager was moved and changed. Rather than relying on it, we simply create a new Document per plot and detach the Document from all existing models.

[jlstevens]

Seems fine to be explicit and I am assuming this has been tested with latest bokeh dev and previous versions.

Both versions access underscore attributes: doc_handler._doc in the old version and now m._document. On that basis we are still relying on internal bokeh API but this version should at least work with the next bokeh release.

Merging.

[jbednar]

Please file an issue on Bokeh asking that there be a defended API entry point to wrap anything we are relying on.