use expm_multiply

I added a pull request to the scipy master branch to improve the support for complex numbers in expm_multiply, and I've also changed this in solve.py in AdiaQC.

Here's the scipy pull request, so that it doesn't complain about imaginary numbers.
https://github.com/scipy/scipy/pull/2495

Using expm_multiply(A, B) instead of expm(A)*B seems to improve the hopfield.py speed from

```
real    0m16.593s
user    0m33.914s
sys     0m59.272s
```

to

```
real    0m13.787s
user    0m23.701s
sys     0m53.507s
```

although I don't really know how to run the AdiaQC tests.