OTsignedFile Update for SignerNymID

This change mirrors a previous change to Open-Transactions, https://github.com/Open-Transactions/Open-Transactions/commit/ebe580720320e6eeb331a7f3fea3039e6cc0802d#diff-d1b85786587be82e5497ce0b18c30ddeR180

Methods for getting and setting a SignerNymID (an optional value), needed in clients such as Moneychanger.