Use low S-value for signatures

The Satoshi client negates the S-value of a signature if the value is above half the order of the curve. The development version of the satoshi client enforces low S-values for signatures. But the stable version (0.9.3) only generates signatures this way, it does not enforce the check yet. From what I understand this change was made to help with malleability and reduce the serialised size of signatures.

See bitcoin/bitcoin#3016 and bitcoin/bitcoin#3637 for more information