Review upgrades.js programmatic interface

[spalladino]

We should review the current Project.createProxy interface, with two goals:

• Remove the need for any global variables or library initialization methods (like ZWeb3.initialize)
• Either make the interface as similar as possible to that of web3 (so it's easy to switch between upgradeable and non-upgradeable deployments), or provide an interface for regular deploys

[pcowgill]

@spalladino Yes, this would be great!

[frangio]

A user reported global state of upgades.js as an issue in https://github.com/OpenZeppelin/openzeppelin-sdk/issues/1504.