[Feature] Issue 245 - Variable Image Width

This is an improved reattempt of [this other pull request](https://github.com/woothemes/FlexSlider/pull/877).

I realised that the mentioned request got stucked due a [merge conflict](https://github.com/woothemes/FlexSlider/pull/877#js-pull-merging) that was never resolved and it was apparently abandoned by the author.

What I've done is to fork the current version, to resolve the conflict and to rebase onto the latest version. 

I've tried to contact the author of the original request before creating this new request, but I've received no reply from him. 

Nevertheless, I suppose that he would be happy if his code is finally added to the plug-in :-)