Improved the output of the getCssGradient method

I changed quite some things in the getCssGradient method, here's the list:

* added the $vintageBrowsers flag in the params, so only the recent vendor prefixes are used by default (but user can still fallback to the old support of needed)
* this is really important: added the standard `linear-gradient` syntax, which is supported by the majority of the browsers today
* removed the -ie-linear-gradient (this thing never existed)
* updated the readme and the demo