Random number functionality and matrix exponentials

[annius]

@scottswift @peddie

Once I'm done with the hackathon I thought I might extend plover functionality for random numbers (could be useful for Monte Carlo tests).

1. sampling from normal distributions using Box-Muller (more efficient in comparison with rejection sampling)
2. sampling from discrete PDFs

Also the following (I have a C implementation sitting somewhere)

• Matrix exponential (using Pade approximations, like MATLAB does), c.f. "19 Dubious Ways..." http://epubs.siam.org/doi/abs/10.1137/S00361445024180