Speed up matVecModM function.

I'm using a lot of random streams like ``uniform(...,nstreams=100000)`` and as I increase the number of ``nstreams``, the compilation time was also increasing. So, I took a look around and found the initialization part of the MRG_RandomStreams uses the ``matVecModM`` function a lot.

What I found is the following line will always be positive (for explanation see below), which means the if-else is wasting cpu time:
https://github.com/Theano/Theano/blob/master/theano/sandbox/rng_mrg.py#L35

I got a 2x speed up when removing the if-else. But, I though it could be faster if we could remove those two for loops. I used some numpy magic and now, on my computer it is 3.2x faster.

The reason line 35 will always produce positive number has to do with the modulo operator: ``x1 % x2 == x1 - floor(x1 / x2) * x2`` (http://docs.scipy.org/doc/numpy/reference/generated/numpy.mod.html) So, as long as the divider (``x2`` in the example, or ``m`` in the ``matVecModM`` function) is positive (which is the case in ours), the remainder will be positive.

That being said, since we are casting the remainder as a int32, the only way ``r`` could be negative is if the remainder was greater than ``2^31-1`` before the cast, which is not the case here because ``m`` is either ``2^31-1`` or ``2^31 - 21069``.