Add copyright and license to GSClient.cpp and GSClient.h
Add support for UDP clients

This makes GSClient an abstract class and adds GSTcpClient and
GSUdpClient subclasses.
Remove const workaround for IPAddress

https://github.com/arduino/Arduino/pull/1798 was merged, so this is no
longer needed. This means that this code will only work with a recent
nightly build, the Arduino 1.5.6 release does not have this fix yet.
Swap if and case

Previously, there was an if containing a case. Now there is a case,
with the if inside each branch. This shouldn't change any behaviour, but
makes it easier to use a different if condition in some case branches
next.
udp
more udp