There are several ways to do this, one of them is to use [Freestyle][1]:

1. In edit mode, select all and press <kbd>Ctrl</kbd><kbd>E</kbd>*> Mark Freestyle edge*

2. Enable *Freestyle* in *Render settings*:  
![enter image description here][2] 

3. Enable *Edge mark* in *Render layers > Freestyle line set > Edge types*:  
![enter image description here][3]

4. Render.

Another way is to use a wireframe material as in [this tutorial][4]: 

1. Add a material and select *Wire*, enable *Z Transparency* and set the *Z offset* up:  
![enter image description here][5]

2. Add another material and assign it to all the faces.

 


  [1]: http://wiki.blender.org/index.php/Doc:2.6/Manual/Render/Freestyle
  [2]: https://i.sstatic.net/Pe9du.png
  [3]: https://i.sstatic.net/ee6TF.png
  [4]: http://www.blendtuts.com/2010/08/how-to-render-wireframes.html
  [5]: https://i.sstatic.net/ZQA8o.png