Obspy integration

Reopening  [PR 61](https://github.com/leouieda/fatiando/pull/61)
*Obspy integration.*

> from fatiando.vis import mpl
> from fatiando.utils import readSEGY
>   
> segyfile = readSEGY('marmousi_nearoffset.sgy')
> mpl.seismic_wiggle(segyfile, scale=0.005)
> mpl.show()


![figure_1](https://cloud.githubusercontent.com/assets/4503573/4017160/04f5f090-2a3d-11e4-9596-6b5858410963.png)


> mpl.seismic_image(segyfile)
> mpl.show()


![figure_2](https://cloud.githubusercontent.com/assets/4503573/4017165/16a003f8-2a3d-11e4-8a0d-bf25410bebc5.png)