Complex Module (Resolves #47)

I've implemented some basic functions:
Stuff yet to be implemented:
- [x]  add
- [x]  sub
- [x] mul
- [x] div
- [ ] pow
- [x] is_zero
- [x] is_one
- [ ] add tests

`compare`/`is_positive`/`is_negative` cannot be implemented as complex numbers cannot be totally ordered

For efficiency, if you want, you can also keep the current `from_two_rats`. That's actually good. And it is then the responsibility of the user to pass in two Rationals.