Make LocalTransactionSigner to sign P2SH inputs.

Transaction inputs are now prepopulated with empty scriptSig. Each signer
is expected to update this scriptSig with a signature at a proper place.
There is a new method in RedeemData to locate index of the key/signature
within scriptSig/program.
To generalize an above approach for all supported types of inputs,
RedeemData can now represent data for any type of input. For
pay-to-address and pay-to-pubkey inputs it is expected to contain single
key and CHECKSIG program.

Signers now accept ProposedTransaction object that could carry additional
metadata shared between signers. For now it shares derivation path of the
signing key.

To preserve the dummy sig feature, a new flag was introduced in a SendRequest.
It specifies whether to fill empty sigs with dummies during tx completion
or not. Default value is true (for backward compatibility).

There is a CustomTransactionSigner class that may be used as a base for
simple third-party signers (or may be not). It is used in unit test which
may be treated as a usage example.