Changed 'Archive' to 'Categories' in entry editor

Noticed that when editing an entry, the categories checkbox container's
name was 'Archive'. I modified the template and the lang file so that
it's 'Categories' from now on.

Before: 
![cats](https://f.cloud.github.com/assets/6784705/2300272/ed709f9e-a0e7-11e3-87c2-20e47db29464.png)
After:
![cats2](https://f.cloud.github.com/assets/6784705/2300273/efa85554-a0e7-11e3-8ee7-b28f6c1c93b0.png)