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.
Use canvas when available - Fixes

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

Use canvas whenever possible (#191)