Detect and prefer Zepto over jQuery

Detect and prefer Zepto over jQuery, update README.md

FitText.js doesn't use any parts of the jQuery API not 
covered by Zepto (zeptojs.com). 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
- update README.md to reflect Zepto support