Added deterministic k + test vectors from Bitcore

Also, there was no check for s being 0, so I fixed that too.

I think maybe the testdeterministicK() function might be better outside of the coinjs.transaction() object... but that's how I wrote it.

Please compare with the following:
https://tools.ietf.org/html/rfc6979#section-3.2
https://github.com/bitpay/bitcore/blob/master/lib/crypto/ecdsa.js#L76

This took me a few hours to write and test, but it's nice to know that everyone can be safer now.

Please try to look for more test cases maybe and run a few tests before merging.