added option 'dontSetWidth' to deal with box-sizing: border-box layouts.

...width(), nor css('width') leads to predictable results...
trying to fix previous errors
somehow changing css twice didn't work, so preparing the css-options before actually applying it to the target
Merge branch 'border-box' of github.com:murb/ScrollToFixed into border-box

Conflicts:
	jquery-scrolltofixed.js
Merge branch 'border-box' of https://github.com/murb/ScrollToFixed into murb-border-box
See #80: Minified the plugin.