add paperjs Raster type

[anotherjesse]

Sample:

  <Group>
    <Raster
      source="http://assets.paperjs.org/images/marilyn.jpg"
      position={[100, 100]}
    />
    <Circle center={[80, 80]} radius={35} clipMask={true} />
  </Group>

[anotherjesse]

@psychobolt I am new to react/js - so I'm not certain if all the places I added Raster were required.

For instance I saw that Rectangle is not included in export type Paper = {... Is that by design or an accident?

[psychobolt]

It is not by design that I didn't implement Rectangle type. As of now flow types are not fully implemented.