Enhanced set dimensions

Added ability to set css & backstore dimensions separately 
Introduced options param to the dimensions setters, options.backstoreOnly & options.cssOnly

When backstoreOnly set to true, only the value of the canvas element width & height will be changed.
When cssOnly set to true, only the css width & height of the involved DOM elements will be changed,
and you should include the unit of measure (px/%/em) with the passed value (eg '100px' '100%') and not just 100
The "closest" I can live with is this: `options.backstoreOnly && this._setCssDimension(prop, cssValue)`