Solidity version updated, refactoring and execute's input signatures need to be passed as v,r,s separate arrays anymore.

[SuperCS92]

Hello, this project is really interesting. I would like to help to maintain the base code up to date. Following is a list of proposed changes with a brief explanation:

• Input parameters uint8[] sigV, bytes32[] sigR, bytes32[] sigS, were replaced by a single array bytes [] memory sig, just to make it more user-friendly. Notice that the signatures are properly split with the internal splitSignature() method.
• Refactor of execute method to separate responsibilities. recoverMulti() now handles the whole validation of signature process.