ECDSA fixes

I added some standard ECDSA test vectors and changed the sign and verify operations to match those described in Suite B Implementer’s Guide to FIPS 186-3 (ECDSA): http://www.nsa.gov/ia/_files/ecdsa.pdf.

For P-192,SHA-256 and P-224,SHA-256 the computed S values don't match the test vectors so I've commented those out. I didn't look into it too much, but a quick check with one of the P-192,SHA-256 test vectors using cryptopp (http://www.cryptopp.com/) gave the same results as this library.