Fixed WhiteNoise process.

Used the method from `nengo.utils.functions.whitenoise`. This is
a fair bit slower than pre-generating the signal, but it doesn't
require knowing the signal duration ahead of time.

Since this uses the same method, I removed the function from
`nengo.utils.functions`, and instead added an `f` method to
StochasticProcesses that returns a function that can be used
as a Node output. In the general case, this just ignores `t`.
In `WhiteNoise`, it uses `t` instead of keeping track of it
internally, which is what `make_sample` does.