Support tokens with callback hooks

[fubuloubu]

From audit:

"Callbacks. Some token standards, e.g. ERC223, ERC721, and ERC777, define callbacks. Some of these callbacks can be invoked upon regular “transfer” calls. This creates a significant security risk in the form of reentrancies. Inside the strategy additional protection measures might be required depending on the token type and its handling. The vault already has reentrancy locks that should be sufficient for most tokens."

NOTE: Currently we do not support other token standards besides ERC20, so it remains to be seen if we would like to support these.