diffie-hellman

pull for #12, still in progress

- [x] modp groups 17 and 18 don't work
- [x] can't generate primes
- [x] ~~we should take advantage of psudo-mersenne primes when possible~~
- [x] different methods on the DH object depending on how it's created

much of the credit really goes to @indutny who did most of the hard word writing an efficient big number library.