Make contract initializable

[ScreamingHawk]

Update the contracts to use initialize functions instead of constructors so that they can easily be deployed as proxies via factories.

Also add factories for ERCXXX presets :)

[drinkcoffee]

This branch has some work towards allowing the ERC721 contracts to be upgradeable or non-upgradeable, and one-time upgradeable.
https://github.com/immutable/contracts/tree/peter-upgradeable-erc721