Add SendRequest.missingSigsMode

Instead of useDummySignatures flag there is now a MissingSigMode enum
allowing to specify what to do with missing signatures.

Available modes:
USE_OP_ZERO - do nothing. OP_0 will be left in place of missing
signature
USE_DUMMY_SIG - insert dummy signature in place of missing sig.
THROW - throw an exception. It would be either MissingSignatureException
for P2SH or MissingPrivateKeyException for other tx types

Default mode is USE_DUMMY_SIG (for backward compatibility).