Support numpy arrays for input in Layer.get_output

Addresses #60, and also adds missing documentation for `get_output` and `get_output_for`.
Well, what I'm actually after is reusing this code without copying it over: https://github.com/f0k/nntools/blob/3de5cbf1d640d9469ba157d52d4074fc6b35cc44/nntools/layers/base.py#L304-315
It only works because we know what the super class looks like. I agree that this is not elegant. Shall we move it into a `as_theano_expression()` helper method instead?