Update FlexSlider.php

fixed short code, make the width/height args optionals (as intended i think); also added fixedWidth option wich calls setCSSWidth (so without the arg its the same as $FlexSlider(1, 400, 300))
Merge pull request #7 from carlos22/patch-1

I just merged it without testing it. I didnt do anything on this module for quite a while so i am no more into it.