Scalar wave modeling

@hbueno let it be known by you.

Master Leo this is a simple scalar wave equation using your classes and methods as base.

I have tested with marmousi data set and have some screenshots to show.

Stuff to discuss:

* Create methods to save simulation on disk from enumerate(timesteps) ( from example elastic_sh cookbook example). Leaving stuff on memory is very heavy and unfeasible for some simulation sizes.
S*U and Madagascar save the simulation in disk BEFORE creating an animation. 

* Create methods to save shot groups (while running simulation?) as segy using obspy. S*U and Madagascar do that as well. 

* Implement (in vis?) trace viewer utilities using Obspy. 

* Create cookbook example using all stuff above. Create a simple shot from marmousi save as segy and visualize it? 

* Implement perfect match layer ? (PML)

Or after the trickier @leouieda [here](http://nbviewer.ipython.org/urls/gist.github.com/eusoubrasileiro/6186494/raw/e0c1153732ae1f13506c0f4b6469fd1d93a000a8/marmousi_fatiando_scalar.ipynb)

![marmousi_movel](https://f.cloud.github.com/assets/4503573/925995/0c4a51f0-ff83-11e2-8a71-f770094eafea.png)
![marmousi_a](https://f.cloud.github.com/assets/4503573/925991/0c44742e-ff83-11e2-88d4-750646b8006f.png)
![marmousi_b](https://f.cloud.github.com/assets/4503573/925992/0c47bfc6-ff83-11e2-9d57-7ae6ed8d7c25.png)
![marmousi_c](https://f.cloud.github.com/assets/4503573/925994/0c483730-ff83-11e2-94c5-91c6863a7391.png)
![marmousi_d](https://f.cloud.github.com/assets/4503573/925993/0c410636-ff83-11e2-9258-b0a64bc2db12.png)