Adding support for complex numbers

Hi,

This may be too raw and unfinished for you, but I added support for complex numbers to the extent I needed it. I apologize that I didn't find the time to add specs as well, but there is most likely still a bit missing and you are the better judge of that. It also appears that my editor removed stray whitespaces which makes the diff much bigger than it should be.

See examples/complex.rb for what works and what doesn't.

In any case, I'm very grateful that you put the effort into this GEM and I leave it up to you to take this, copy parts, ...

Cheers,
-max