A divide operation

A divide operation is very common for a calculator. Therefore we would
also like to have one.

Change-Id: I6dcbfa111a72994190633466b504c127b5a6ed59
Se edita el fichero Calculator.java

Change-Id: I3696dd750756f8c5cca3d23521161cf44dd183f1
Se edita para subirlo como peticion de cambio. Se ha corregido.

Change-Id: Ia9485ed7166d05a75208d6ebd3ee179574bcc26f