Check signature nonces for validity

The documentation implies that this check is happening, so make it so.
Without this check, passing an invalid nonce will trigger an internal assertion.
Merge pull request #9

ac274bf Fix x64 gmp init on platforms that define UL as 32 bits. (evoskuil)
Merge pull request #8

ba8fc0e Check signature nonces for validity (William Swanson)