Multithreaded gravmag.sphere with OpenMP

Using Cython prange function in gravmag.sphere yields a 2x speedup in my
i5 processor with 4 threads on a (200x200) grid. On smaller grids
(20x20) it is at worse the same speed.

Had to separate the kernel Cython functions because I couldn't do
"elif field ==" without the GIL.

Putting the gz kernel on a cdef inline function actually made it much
faster.
Setting OpenMP number of threads for Travis

As per this issue: https://github.com/travis-ci/travis-ci/issues/1553
OpenMP support for gravmag.prism

Had to change the coordinate system shift (x[0] = x2 - xp[l]) to use
dx = x[i] - xp[l] inside the integration loops because I can't alter the
x array in parallel.

Benchmarks show 2x speedup in an i5 processor with 4 threads.
harvester doesn't seem to be much faster though.
Merge branch 'kernel-functions' into kernel-openmp

Conflicts:
	setup.py

Changed the setup for building extension modules. Getting a list of
module names plus extra compile args.
Merge branch 'kernel-functions' into kernel-openmp
Benchmark prism and sphere against numpy code
OpenMP for gravmag.polyprism

Benchmarks are very good! Sometimes more than 2x speedup over numpy
version.
Moved the vertices loop out of kernels to avoid segfaults.
Had to change the way PolygonalPrism makes .x and .y attributes so that
they are C contiguous arrays.
Instructions for OpenMP support on Windows
Updated changelog for OpenMP support in gravmag
Merge pull request #106 from leouieda/kernel-openmp

Add OpenMP support to forward modeling modules in gravmag.