Removed the neprism module

This was a test module using numexpr to forward model prisms in gravmag.
Didn't give adequate speedups and wasn't worth adding as a dependency.
Add Cython .c files and break build in 2 parts

Separate the generation of .c files by Cython and building the C
extensions. setup.py now takes a --cython argument that triggers Cython
conversion. If not set, will just build the given .c files.

To avoid problems with readthedocs, modules try to import the Cython
mods and upon failure substitute functions with a mock that raises an
exception.

Docs of gravmag.prism is now mostly in the module docstring because all
functions are in the Cython module. Can still get the docstrings of the
functions but not in the built docs.

TODO: Need to include the .c files in Manifest
Include .c files in MANIFEST

Needed so that the Cython generated .c files are included in source
distributions.
Merge branch 'master' into cython
Merge branch 'master' into cython
Build before testing in Makefile

Added build as a dependency for test
Removed cython and mayavi install in Travis

They were unused. Also moved some things around in the commands.
Sphinx build had to be before install in Travis

Because the build fails with the compiled binaries.
Set the virtualenv flag for Travis again

This time I added comments so that I don't forget why it was there.
Merge pull request #75 from leouieda/cython

Include Cython generated .c files in the repo