Add author and date to log #384

This commit adds the commit date and author to the log view. This should resolve issue #384. I'm not overly familiar with elisp or the magit code base: if you're interested in the patch, let me know if you want me to change or fix anything.

This is how it looks:

![Screen Shot 2013-01-26 at 17 33 57](https://f.cloud.github.com/assets/150883/100017/4b59b99e-67df-11e2-89b2-5bfc0d243d49.png)

Thanks,

Greg