Ensure all transaction builder fns can return unsigned equivalents

[kyranjamie]

Requirement Similarly to #112, this library needs to expose a way with which unsigned transaction equivalents can be created, and signed elsewhere.

One way to achieve this might be to have all tx builder functions, makeContractCall in this instance, to optionally accept senderKey, and return an unsigned transaction.

senderKey is currently a mandatory property in ContractCallOptions.

Context To participate in Stacking, the stacks-wallet needs to use the contract call method in two scenarios, with a:

1) Software wallet 2) Ledger wallet

makeContractCall cannot be used for Ledger wallets. We first need the unsigned transaction to sign on the device, then broadcast

@hstove's put together a client to handle PoX you can see here https://github.com/blockstack/stacks-wallet/pull/258.

[agraebe]

@kyranjamie I see the UserX sprint associated with this issue. do you plan to work on this issue?

[hstove]

Closing because it's all done and merged! 😀