Detect and prefer Zepto over jQuery

Detect and prefer Zepto over jQuery

FitText.js doesn't use any parts of the jQuery API not 
covered by Zepto (zeptojs.com). Updated jquery.fittext.js 
to detect and prefer Zepto over jQuery and renamed to 
fittext.js. Now FitText can be dropped into mobile projects 
using Zepto or jQuery projects without code changes.

- detect and prefer Zepto in immediate function invocation
- rename jquery.fittext.js to fittext.js
- update author's comment