Single commit of child window fixes

[nielsenb-jf]

This pull request fixes the issues noted in issue 702. Essentially, if Plotly was provided an element in a different window or tab, the plot would not be styled correctly and interactivity (hovers, drag zoom, etc.) would not work.

It is squished to one commit due to issues caused by multiple merges during development see this previous PR.

The following are the big changes to fix this issue:

• Graph divs now keep a reference to their 'parent' document in the `_document' property.
• Styles are now injected to the parent document on the first call to Plotly.plot.
• Anything that draws over a document for interactivity purposes (eg. dragcovers), now uses the _document reference to ensure it is drawn in the correct document.
• Notifiers now require a graph div as the first argument so that notifications are rendered in the correct place

[etpinard]

Hmm. That commit message was meant to be temporary. Can you git commit --amend it? Thanks.

[nielsenb-jf]

Try this PR.

[etpinard]

In the future, you can git push -f instead of opening new PRs every time.