Fixed tanh/sigmoid types

Issue: FastDropoutRnns with `theano.config.floatX='float32'` not working (at least to me, with default theano settings). That is because of the operation done in the "recurrent layer". More concretely, `tanh`, and `sigmoid` always returned `float64`. Casting `PI` to `floatX` solves the problem.

Curious, because PI is only assigned at the beginning with `PI = np.array(np.pi, dtype=theano.config.floatX)`, so it should be `floatX` already. Even more curiously, the debugger states that `PI.dtype` is `float64` whereas `np.array(np.pi, dtype=theano.config.floatX)` is `float32`.