Implement convolution of PDFs

[ibab]

Theano implements fast convolution via FFT. We could implement a Conv distribution that takes two distributions and represents their convolution.

[ibab]

You would use it like Conv(x, dist1, dist2). If either dist1 or dist2 don't contain the variable x, we should throw an error.

[ibab]

There may be distributions that we can convolute symbolically. If that's the case, Conv should automatically use the fastest approach.