Implement numerical evaluation of products

On Fri, Jun 27, 2014 at 10:08:45AM -0700, Aaron Meurer wrote:
>    What number is this? Product.doit() can't do it.

This is an expansion of sin(pi*x) for x=1/2, except for pi/2 factor.

See e.g. http://en.wikipedia.org/wiki/Infinite_product#Product_representations_of_functions