Use canvas whenever possible (#191)

Drawer uses an HTML5 canvas element if it's available.

Viewer.useCanvas can be used to override (default is true).
changelog.txt update
Merge remote-tracking branch 'origin/master' into canvas-detect

Conflicts:
	src/openseadragon.js
Use canvas when available - Fixes

Updated documentation doclet.
Enclosed detection code in a function.
Eliminated the Drawer USE_CANVAS global and replaced it with a
Drawer.useCanvas instance variable.
Added preventDefaultAction flag to MouseTracker

Added a boolean preventDefaultAction property (default false) to the
event object passed to MouseTracker handler methods. Implemented in the
following MouseTracker subscribers:

* Viewer.keyboardCommandArea.innerTracker.focusHandler:
preventDefaultAction == true prevents scrolling viewer into view
* Viewer.keyboardCommandArea.innerTracker.keyHandler:
preventDefaultAction == true prevents viewer keyboard navigation
* Viewer.innerTracker.clickHandler: preventDefaultAction == true
prevents viewer zoom on click
* Viewer.innerTracker.dragHandler: preventDefaultAction == true prevents
viewer panning with mouse/touch
* Viewer.innerTracker.scrollHandler: preventDefaultAction == true
prevents viewer zooming on mousewheel/pinch
Use canvas when available - Fixes

Removed redundant useCanvas property from drawer
MouseTracker preventDefaultAction Unit Tests
Merge pull request #268 from msalsbery/canvas-detect

Use canvas whenever possible (#191)
Merge branch 'MouseTracker-preventDefaultAction' of github.com:msalsbery/openseadragon into msalsbery-MouseTracker-preventDefaultAction

fixed Conflicts:
	changelog.txt
Changelog updates