New Less variable for Image path to ease integration of Onyx Less files

To help fix ENYO-2630, a new Onyx `@onyx-image-path` variable was introduced in `onyx-variable.less`. This variable contains the path of the image directory relative to the location of the compiled CSS file.

For Onyx, this variable is set to `'../images'`

For any application directly importing Onyx Less files, this variable will have to be overridden to specify onyx image path relative to the application's CSS file.

For instance, Ares will override this variable with '../lib/onyx/images' in ares-variables.less

Onyx-variables was changed to use `@onyx-image-path` in each `url()` spec. The only side effect on the compiled css file is a change of the url() spec: the url is now quoted.

This was tested on Chromium 27 and IE 8-10

* image-path-in-less: clarified comment (HEAD, origin/image-path-in-less, image-path-in-less)
* image-path-in-less: recompiled css. url() call parameter is now quoted by lessc
* image-path-in-less: use @onyx-image-path for all url() calls
* image-path-in-less: use @onyx-image-path for all images
* image-path-in-less: use @onyx-image-path for TabBarItem image



Enyo-DCO-1.1-Signed-off-by: Dominique Dumont <dominique.dumont@hp.com>