Add function for generating keypair from a seed

[mithrandi]

I ended up using the libsodium version of this function, added here.

A few comments:

1. I added this in two commits, one for the underlying C function, and one for the bindings. Let me know if you want me to squash these into one.
2. I didn't add a test because... well, I'm not really sure how to test this (also I can't run the tests at the moment, but the other two pull requests open at the moment fix this). If you can think of something, I'd be happy to try implement it.

[mithrandi]

Oops, had the wrong bug number for some reason; fixed the reference in the commit message.

[thoughtpolice]

Thanks. I want to review this manually first (Since I think libsodium's ed25519 may be slightly different; there are a few 'ed25519 key formats' floating around in some places I think, an unfortunate side effect of no real standard.) Clipping the C code out is fine, and keeping it in a separate commit is as well.

[thoughtpolice]

Oh, nevermind. I was thinking of the actual format of output messages, not keys themselves. Yep, this looks OK to me.