Add ERC20 support

[JasoonS]

A first step would be to make a version of all the code that works with ERC20 tokens.

A next step would be to allow both Eth and ERC20 tokens. (maybe this can be skipped if we just use wrapped Eth, but that definitely is not as nice for UX)

It would be nice for example to allow users to use cETH rather than ETH as collateral so they could try earn interest on their deposit at the same time.

Finally, we could allow users to set price/deposit in any token. With this it would be great to plug into a decentralized exchange like Uniswap and allow users to purchase a collectable with any asset too.