made baseUrl (https://api.xing.com/v1/) configurable

Made the baseUrl configureable during the init of XingTemplate.
Current implementations should remain unaffected.

Main motivation is that we can use the libary in XING internal test environments as well.

P.S. I would find it very helpful if we could agree on a code format strategy. For example AbstractTemplate uses spaces and tabs for indentation.
We should try to be downwards compatible and keep the ConnectionTemplate(RestOperations restOperations)  as well