Added resource monitoring and string formatted dating to __repr__

Sebastian,

I really like you package and use it pretty frequently at work. In the course of using it I thought some resource monitoring would be really nice as I often let things run and come back and check on them long after they have finished. I have had this functionality in my local pyprind package for a while now and thought it might be worth merging it in.

Changelog
---------------
-changed end attribute of Prog to a time.time instance, instead of a time delta
-added a total_time attribute to Prog to serve the functionality of the old end attribute
-added psutil dependency to allow for resource monitoring
-added several new objects to the __repr__ function with the above changes (see attached image)

If you do not like the step of adding a non-standard dependency (psutil) then take a look at the other branch on my forked repo (grromrell/optional_psutil_update). There the psutil functionality is only used when explicitly called for.

![image](https://cloud.githubusercontent.com/assets/6600690/2745805/56b16fb4-c73b-11e3-8f93-e38350e33ba6.png)