Inherit from Arduino Stream Class

By inheriting from the Arduino Stream class the SDI-12 library now has access to the parseFloat() and parseInt() functions. This allows the Arduino to more easily manipulate incoming data. The compiled size of the library is not significantly changed. There are no known cons or downsides to this change. 

An additional example has been added to demonstrate the functionality. 

Please review the Stream branch to see if it is ready to commit.