Signature-based minting restriction

[Hadrienlc]

close #3 based on https://github.com/liteflow-labs/rarible-protocol-contracts/pull/4

[NicolasMahe]

Need to check with Rarible dev:

• Is it ok to add the signature to the data.signature payload. No breaking change from the smart contract view.
• Do they prefer to add a new function that accept a another parameter "operator signature"? Even it is requires a lot more changes in the rest of the code base to implement it.