Cythonize Dispatcher methods

This is currently a quickly implemented proof-of-concept to cythonize `Dispatcher.__call__/resolve` as discussed in #13.

We will make Cython and a C compiler optional.  We also still need to use Cython for `MethodDispatcher`.

@mrocklin, I'm curious how much faster this is for you.  I have not performed any benchmarks.  I believe we can make `__call__` 50-100% faster than what is currently done in this PR, but I want to make sure I'm on the right track.