Add input value field to wire.NewTxIn

[jrick]

One of the properties of the signature hash algorithm proposed by #950 is the unconditional commitment of the previous output amount (removing the need for SigHashAllValue). As this makes knowing the previous output amount required during transaction creation, the wire.NewTxIn function should be modified to take the value as a parameter.

[dnldd]

I'd like to work on this.