Added end() function to Soft Serial - fix for #463

[maidnl]

This PR introduces the end() function to the Soft Serial library. Using the end() function allows to use the pins allocated for the Soft Serial "manually" as GPIO. Soft Serial can be enabled again with a new call to begin(). This PR has been tested using Soft Serial pins as GPIO (so simple Digital Input or Digital Output). Please note that when 2 pins are allocated to Soft Serial it is not allowed to change the function associated to these pins to "complex" configuration like PWM or PIN with Irq since the needed resources are taken by Soft Serial function.