Added pixelRatio option to override canvas upscaling. Resolves #876, test included...

Added pixelRatio option to override canvas upscaling. Resolves #876, test included. (#877)

* Added pixelRatio option, using where appropriate.

Falls back to old behavior if not specified.
Added to options reference,
Added to Dygraph.prototype.resizeElements_,
Added to rangeSelector.prototype.updateVisibility.

* Fixed bug in range-selector pixelRatio implementation, tests pass

* added test for the pixelRatio option, properly run.

First fails without change, then passes after change.

* Added pull request suggestions

Fix typo in docs, documented a float type,
Simplified method of reading option.

* Added note about pixelRatio affecting resolution