Customisable socia media card image

### From
![screen shot 2014-09-12 at 17 41 32](https://cloud.githubusercontent.com/assets/1492162/4253083/eba50f1c-3a9b-11e4-8f77-41248d348353.png)

### To
![screen shot 2014-09-12 at 17 42 12](https://cloud.githubusercontent.com/assets/1492162/4253084/efa72398-3a9b-11e4-94fb-0e34ba88788d.png)

I don't like default arguments at all. They don't play well with passing around functions as values (scalac is easily confused, poor thing).