Include begin procedure and change object constructor code about hx711

[CFTechno]

Please see the remarks made by Paul Stoffregen in
http://forum.arduino.cc/index.php?topic=45794.0
and
http://forum.arduino.cc/index.php?topic=78114.0

Setup() is the only moment within a sketch dat you can be sure that all needed initialization is done.

[SirUli]

@electrokean when you want to wrap the library like shown here for other sensors, then this is very useful 👍

[bogde]

looks like a good idea, i'll think about it and let you know.

[electrokean]

@CFTechno can you provide an example of when pin control wouldn't be available at the time of global object creation? I don't see a problem with the change other than backwards compatibility, I am just trying to see what value it adds and in what circumstances.

[amotl]

Just wanted to let you know that by [1], we let the constructor-based initialization go completely. So, it can't even be used by accident anymore.

[1] https://github.com/hiveeyes/HX711/tree/spring-cleaning