Support numpy arrays for input in Layer.get_output

Addresses #60, and also adds missing documentation for `get_output` and `get_output_for`.
I like that! Seems like it could be useful to have this functionality packaged for people who want to implement their own layers.

Also, `as_theano_expression` could be made to accept both existing Theano expressions and numpy arrays, so that the type check is inside the function as well (but I guess that's what you were planning to do anyway).