Added icon font instead of base64 image

Externalized js and css + added social icon font instead of using
base64 image.

I decide to externalize the js and css to make it a bit clearer for myself and to make it easier to implement in my project.

I also replaced the base64 image with an icon font to get a sharp and clear vector based twitter-icon.

Not sure if think this is an enhancement but I leave that up to you to decide.