Support for external signers

[zorba128]

Library should support scenario with signing process detatched from transaction building:

• transaction gets prepared
• it gets serialized without signatures and delivered to signer component
• signer generates and returns signatures
• signatures are attached, transaction is broadcasted

To support that scenario:

• Message.setFeePayer should accept public key only (private key is not needed at that stage)
• Transaction.sign should allow adding signature to already constructed message
• Message.deserialize and/or Transaction.deserialize is needed

Actually there are few more issues that might be cleaned up

• note Transaction and TransactionBuilder provide same functionality. I'd suggest removing mutation methods from Transaction, and keep only signing and serialization logic there.
• Transaction.sign mutates underlying message (setFeePayer); rather than that, it should just sign whatever is already there; if fee payer needs to be applied, it might be done during building stage
• note the Transaction actually might operate on serialized message; this would also solve its problematic lifecycle, with NPE thrown if serialize is called without sign()

See #78 - it probably was requested for same reason

Take a look at recent commits at my fork: https://github.com/zorba128/solanaj/tree/signatures

[zorba128]

For now I think it will be hard to fix all the issues with Transaction class without redefining its responsiblity.
I'd suggest to merge my changes to Message - it should allow anyone to develop own mechanics around it, not breaking api compatiblity. Later we could try to define new interfaces for building/signing.

[zorba128]

See #84