[1-wire] onewire.connector should be exposed

[gen2thomas]

This would simplify the usage in custom applications. Currently the application have to copy the interface, e.g. like so:

// onewireConnector lets adaptors provide the drivers to get access to the 1-wire devices on platforms.
// TODO: can be removed if gobot is fixed
type onewireConnector interface {
	// GetOneWireConnection returns a connection to a 1-wire device with family code and serial number.
	GetOneWireConnection(familyCode byte, serialNumber uint64) (onewire.Connection, error)
}