refactoring MRBrewOperation with improved api, breaking backwards compatibility

- renamed 'operation' property to 'name' (no more ambiguous [operation operation] calls!)
- renamed initWithOperation:formula:parameters: to initWithType:formula:parameters:
- renamed initWithOperationString:formula:parameters: to initWithName:formula:parameters:
- renamed operationWithOperation:formula:parameters: to operationWithType:formula:parameters:
- renamed operationWithStringOperation:formula:parameters to operationWithName:formula:parameters:

phew!