Releasing 2.1.0

Add timeout to prevent lock-up

Prevents lock-up when waiting for echo return. Delegated Ultrasonic constructor for three-pin operation

The timeout is an optional parameter (20000UL by default) in the constructor or defined at runtime.

On the constructor:

Ultrasonic ultrasonic(12, 13, 40000UL);

Or in runtime:

ultrasonic.setTimeout(40000UL);

Tanks @eliotlim