Adding uninstall description pkginfo key

As discussed in https://groups.google.com/forum/#!topic/munki-dev/zC03ac1jr6, I want to have an uninstall description to let users know the reason for the uninstall.  I've made the change locally on my machine and verified that the uninstall description is correct in MSC. 
![screen shot 2014-10-23 at 2 32 04 pm](https://cloud.githubusercontent.com/assets/6855782/4762187/da4274ac-5afe-11e4-8e2e-7e21d1868faf.png)