Adding Data URI scheme support

Add support for [Data URI](http://en.wikipedia.org/wiki/Data_URI_scheme), which is widely used in web development.

Support following content type:
* image/png
* image/jpg
* image/jpeg
* image/gif

Usage:

    Picasso.with(this)
                .load("data:image/png;base64,iVBO...")
                .into(imageView);