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.
Added links to pull requests in changelog

Filled in missing entries and linked the entries to the corresponding
PRs that implemented them. Also put links to the ones that the team made
for the first time.
Merge pull request #75 from leouieda/cython

Include Cython generated .c files in the repo
Convert initial guess in solvers to array

This way I don't have to pass arrays all the time as initial (lists will
work fine as well).
Merge branch 'master' into inversion
Optionally use the readthedocs theme

Try to import the readthedocs theme. If fails, use the default one.

Theme available here: https://github.com/snide/sphinx_rtd_theme
Merge branch 'master' into inversion
Converted gravmag.basin2d to new framework

Implemented in 2 classes: Triangular and Trapezoidal. Both have a method
topolygon to convert an estimate to a mesher.Polygon.