Implementation of 3D quiver

Hi,

We are a small team of student from University of Toronto. Our instructor asked as to implement a MPL feature as one of our project :) so we choose 3D quiver, as mentioned in #1026 and [mailing list](http://www.mailinglistarchive.com/html/matplotlib-users@lists.sourceforge.net/2013-03/msg00034.html)

We only had a week to work on the implementation and term report, so it's not a full featured quiver as 2D quiver. But we got the basics working and it should be usable.

![figure_2new](https://cloud.githubusercontent.com/assets/4098537/2648480/e290a0c2-bf57-11e3-89c5-9e12b91d6c2e.png)