MRG: add infomax

cc @agramfort @fraimondo

still testing. works already fine but trying to understand some details with @jdammers.
```

+def _sigmoid(t, copy=True):
 +    """Helper function: return 1. / (1 + np.exp(-t))"""
 +    if copy:
 +        t = np.copy(t)
 +    t *= -1.
 +    t = np.exp(t, t)
 +    t += 1.
 +    t = np.reciprocal(t, t)
 +    return t
```