ENH: Alex Leach's wrapper of Roger Sidje's EXPOKIT

Add the EXPOKIT wrapper, with some benchmark and testing code.

Through testing I've found that the expm provided by the wrapper fails for a matrix full of zeros (causing a fortran `stop`) and for the following matrix mentioned by Cleve Moler (returning a matrix full of `nan`s).
http://blogs.mathworks.com/cleve/2012/07/23/a-balancing-act-for-the-matrix-exponential/

My benchmark times were a lot faster than the ones reported in https://github.com/scipy/scipy/pull/354.  I'm on some 6-core Xeon with openblas, which I would have thought would be comparable to the system used for the other benchmarks.  Maybe I am doing the timings wrong, or maybe the numbers reported in the other PR were using a fake BLAS or LAPACK instead of actually using MKL.

```
Times averaged over 1000 iterations
Array size               EXPOKIT times            SciPy times              
5                         0.000 += 0.000            0.002 += 0.000           
10                        0.000 += 0.000            0.002 += 0.001           
15                        0.000 += 0.000            0.003 += 0.002           
20                        0.000 += 0.001            0.003 += 0.001           
25                        0.000 += 0.000            0.003 += 0.001           
30                        0.000 += 0.000            0.003 += 0.001           
35                        0.000 += 0.001            0.004 += 0.001           
40                        0.000 += 0.000            0.004 += 0.001           
45                        0.000 += 0.001            0.004 += 0.001           
50                        0.000 += 0.001            0.005 += 0.002           
```

Sorry I screwed up the first commit message, and then I tried to go back and change it somehow, but this only added more commit messages and caused some merge to happen.