Screenshot enhancement

See #10 and [screenshot proof of concept](https://gist.github.com/anthonyurena/10069110). I figured it out!
I wasn't sure where this would go in the project (if at all) or how it would be integrated, but here's my best guess.

I'm 100% willing to clean things up and make a complete overhaul of it. The `screenshot!` function in particular could use some work in terms of flexibility. I figured it would be beneficial to open a pr since #10 is sort of irrelevant now that I've figured out what I wanted to accomplish.

The proof of concept (and code in the pr) can be used from the repl with the play-clj-example games:
![taking a screenshot from the repl](https://cloud.githubusercontent.com/assets/5338832/2639622/0904d892-becf-11e3-9518-bc1add05580c.png)

**Result**
![1396928162581](https://cloud.githubusercontent.com/assets/5338832/2639629/37e6771a-becf-11e3-8c8c-9a7200eac8ca.png)