add intrinsic plasticity

A learning activation function that models intrinsic plasticity (plasticity of 
the soma). Learning happens on-line by stochastic gradient descent.
The activation function learns to produce activations that follow an exponential
distribution.

Intrinsic plasticity is unsupervised reservoir tuning. The learning can be turned
off at any time.
Merge branch 'master' of https://github.com/robertpfeiffer/reservoircomputing