`op.get_params()` exists. Now it checks if `ops.params_type` exists.

Add a default implementation of `Op.get_params()` which tris to detect
and filter params from the op if `op.params_type` is defined with a
Wrapper, else it will raise a `MethodNotDefined` exception.

Rewrite test_wrapper. The tested op now uses 3 theano types: TensorType,
Scalar and Generic. Test methods are also renamed and rewritten to be
more readable.

Simplify `Wrap.__hash__`, as `Wrap` is now considered as internal. The
ndarrays are now hashed first with `theano.tensor.utils.hash_from_ndarray`.