Mime Backpack

This is just a small commit specifically aimed towards the mime. 

This adds sprites for a mime backpack(both for the mob and the storage), and the small bit of code for this new backpack and for the mime to spawn with it.

I have tested this on a local server and everything seems to work fine.

![mimepack storage](https://cloud.githubusercontent.com/assets/4686269/2569473/24b74918-b8ed-11e3-9192-0c3ea0e1ff4b.png)

![mimepack mob](https://cloud.githubusercontent.com/assets/4686269/2569521/b49fe2ba-b8ed-11e3-8fca-6fd0ae8b2cca.png)