Test for numba performance improvements

Hi all,

Doing some procrastination this afternoon, I thought I'd take a look at the numba package and how easy it is to use.  If you have not heard of it, numba is an attempt to make a JIT (just in time compiler) for python with numpy support.  In practice, this means that a lot of python code can be dramatically sped up.

To test this, I put a jit wrapper around the Debye function (some of the most time consuming code in burnman) and immediately saw a 2-4x speedup for code that evaluates the EoS a lot.

The downside of this is, of course, that it introduces an additional dependency in numba+llvm.  In this branch I've set it up so that if numba is installed, it uses it, otherwise it falls back on the pure python approach.

Thoughts?
Ian