Update tokens, contract names, tests

[drewstone]

This PR:

• renames contracts to be more concise
• changes directory structure up a bit
• removes ERC20 anchor, opts for wrapper anchor
• allows any user to wrap/unwrap and allows external contract to wrapFor and unwrapFor (e.g. anchors)
• simplifies minting/transfer on _processWithdraw to just mint if balance is less than denomination
• adds constructor params for specifying addresses of bridge, admin, handlers
• updates tests