Adding scalar wave equation.
uninstalled fatiando and added few lines to make call to new method work
(in cython)
Scalar wave equation tested working with madagacar marmousi data set.
Very similar with elastic_sh. 
Removed factor multiplying source to have expected energy from source in
simulation.
Fixed errors. Missing cookbook example.
Added gauss derivative source and started creating cookboook example for
scalar wave equation.
Merge remote-tracking branch 'upstream/master' into andrebranch

Conflicts:
removed	fatiando/seismic/_cwavefd.pyx I will have to fix the calls also after
simple merge	fatiando/seismic/wavefd.py I will have to fix the api calls after
scalar wave equation and cookbook example ready.
Merge remote-tracking branch 'leouieda/master' into andrebranch
Applied suggestions: maxdt, calls to mpl, function definition to be
compatibible to other simulations in fatiando and better description to
cookbook.
Spellcheck wavefd and minor tweaks to Andres code

Corrected a lot of spelling error (most my own).
Removed some blank lines, wordwrapped some to 80 chars and made the
docstring of scalar_maxdt a raw string.
Also cleaned up some old invalid comments.
More tweaks to docstrings in seismic.wavefd

Added entry for scalar_maxdt in function list.
Better formula for GaussSource.
Changed old wlength in MexHatSource to frequency.
Exchange numpy.ndarrays for memoryviews in wavefd

Changed the ndarray buffers in the scalar wave simulation for the newer
memoryviews in Cython, e.g., double[:,::1]. Supposed to be faster and
more updated syntax.