Improved URL encoding

I've borrowed [svg-url-loader](https://github.com/bhovhannes/svg-url-loader) encoding function to achieve an optimized URL encoding.  
The diffreneces can be seen in [this blog post](http://codepen.io/tigt/post/optimizing-svgs-in-data-uris).

See merge request !2

Readme fixes

I've fixed some typos, and corrected the usage example to be inside a gulp task.

See merge request !1