Add Obj-C syntax highlighting to README.

Another tiny PR cause I'm in the mood to tidy up. :smiley:

Looks like this now:

![screen shot 2014-02-23 at 7 46 32 pm](https://f.cloud.github.com/assets/504184/2242538/4e34d8b2-9d06-11e3-8712-453ed28759dc.png)