Add matrix multiplication infix operator

[ColCarroll]

This adds support for @ matrix multiplication. Also fixes #3618 once we cut a release 😁

Note this is a bit of overkill, in that I just wrap theano's TensorVariable, and defer multiplication to tt.dot. I could instead add these methods to each class, but this felt better.

I am not sure if I should also update pm.data.GenTensorVariable -- I decided not to, since minibatch matrix multiplication seems... tricky.

[fonnesbeck]

Good idea! I See no problems with the implementation.

[fonnesbeck]

Doesn't have to be here, but maybe we can replace some tt.dot calls in the notebooks to make folks aware this works.

[ericmjl]

Hitting merge!