Better Cython in seismic.tttime2d

Replaced lists for memoryviews of numpy arrays.
Replaced lambdas for an inlined cdef function.
General speed up of 300x. Yeah. My old code sucked.
Renamed _cttime2d.pyx to _ttime2d.pyx

Just to follow the new name convention for Cython modules.
Doesn't really make sense to have a python alternative to many modules
because they wouldn't run in a reasonable amount of time.
So modules that have no alternative _*.py module don't need the 'c' in
the beginning.
Removed parallel fwd modeling from srtomo recipes

Don't need it because of new speedups.
It's not longer a bottleneck as the serial version is 300x faster than
the old parallel version.
Updated changelog

Included entry on speedup of seismic.ttime2d
Fixed wrong link tag in seismic.ttime2d docs

It read fatiando_seismic_ttime. Missing the 2d
Merge pull request #62 from leouieda/traveltime

Better Cython for `seismic.ttime2d`