Write new_block method for Blockchain class

We need to code the new_block method for our Blockchain class. This method should create a new block in the blockchain. The parameters will be 'proof' and 'previous_hash', which should be set to a default value of 'None'.

This method should also create a 'block' dictionary that contains the following keys:
'index' which should be the next item on the chain list
'timestamp' which should be the time (you will need to import and use the time library)
'transactions' which should be the current_transactions list
'proof' which will be the parameter proof
'previous_hash' which will be the parameter 'previous_hash'

The method should then reset the list of transactions to an empty list, and append the block to the list 'chain' before returning the block.