SVG sprite optimisation: urlencoded

Take 2 on the #4381 PR

- Sprites are URLEncoded (should work in all browsers)
- Compiles well on Ruby 1.8.7

### Gains

**Before:**_global-icons-svg-base64.scss 107KB
**After:** _global-icons-svg-utf8.scss 100KB

**Before:**_global-icons-svg-base64.scss.gz 28KB
**After:** _global-icons-svg-utf8.scss.gz 22KB

**Before:** _commercial-icons-svg-base64.scss 31KB
**After:** _commercial-icons-svg-utf8.scss 26KB

**Before:**_commercial-icons-svg-base64.scss.gz 10KB
**After:** _commercial-icons-svg-utf8.scss.gz 6KB

It would be great to have a cross browser testing session on this branch @mstorijo 

![ ](http://gifs.joelglovier.com/oh-shit/refreshing.gif)