Add dependency management

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There is no way to deterministically build this project using the correct dependency versions. I recommend adopting dep and ensuring the following dependencies are constrained:

• github.com/OpenBazaar/wallet-interface constrained to ethereum-master
• github.com/OpenBazaar/multiwallet constrained to ethereum-master
• github.com/OpenBazaar/openbazaar-go constrained to ethereum-master
• github.com/OpenBazaar/spvwallet constrained to ethereum-master

If dep is used, the following prune options must also be used so non-go files do not get removed from ensured dependencies:

[[prune.project]]
  name = "github.com/ethereum/go-ethereum"
  unused-packages = false

[[prune.project]]
  name = "github.com/karalabe/usb"
  unused-packages = false

In order to know this is working, we should be able to include this package in a fresh go environment and run one (or a few) commands to give the project the dependencies (in their appropriate version) it needs.