Grab bag of fixes and usability improvements resulting from demo app development

Grab bag of fixes and usability improvements resulting from demo app development:

* enyo.Collection
	* Add `isFetching` property, for showing loader spnner via binding
* sources/xhr.js
	* Separate jsonp and ajax request options, so callbackName can be passed
* enyo.Image, ui.css
	* Add `sizing` property, which renders image as `<div>` with background-image, and allows use of `contain` or `cover` background-size properties
	* Add `position` property for use with `sizing` property, for setting background-position
* less:
	* Use relative URL's when compiling less files.

Enyo-DCO-1.1-Signed-Off-By: Kevin Schaaf (kevin.schaaf@lge.com)