Support #0 in pure functions

This is a fix for issue #5 - adding support for pure functions with `#0` slots.

I used the example given in the bug report for the documentation, but it looks similar to one on http://reference.wolfram.com/mathematica/ref/Slot.html - I'm not sure how similar to the MMA documentation we're allowed to be. I'll come up with something different if this is too close.

Thanks!