Make f.fn.allow_gc accesible with the CVM and allow to change it dynamically

@RoyXue I found this problem with your PR

python -c "import theano;v=theano.tensor.vector();f=theano.function([v], v+1); f.fn.allow_gc=False; f([1]); f.free(); print f.fn.storage_map"/Tmp/lisa/os_v3/lib/python2.7/site-packages/scikits/__init__.py:2: {Elemwise{add,no_inplace}.0: [None], <TensorType(float64, vector)>: [None], TensorConstant{(1,) of 1.0}: [None]}

Here, we see that the TensorConstant storage was set to None. You should not change Constant storage map.