Polynomials on tetrahedra, documentation via doxygen

There is also a new example on the reference triangle.

Windows support through Appveyor

As well, the docs are made available as a PDF, the segmentation fault reported in #32 is fixed, and there is some code cleanup: static functions are not prototyped; vecnorm => norm; and, the tests are partially reformatted.

New Travis CI script and colours for examples

There are also minor bug fixes to ensure typed pointer casts and the code conforms to the gnu99 dialect.

Harmonic polynomial transforms on the disk, sphere, and triangle

This introduces the first iteration of the public API, consisting of computational kernels and driver routines for the harmonic polynomial connection problem and FFTW-based synthesis and analysis on tensor product grids. There are two examples, illustrating the spherical harmonic addition theorem, and the integration of holomorphic functions on the disk. Preliminary work on spin-weighted spherical harmonics is included, as well as support for components of a vector field in the tangent space to the sphere in spherical coordinates.

This is a joint effort with @dawese. Thanks to bug reports by @dlfivefifty and @ahbarnett.

Happy transforms!