Add a first version of setConfigXML

Without tests for now, but with an example file.

Also includes ordering the parameters in the URL. If not order, it will fail
when calling setConfigXML. More discussion about it at:
https://groups.google.com/d/msg/bigbluebutton-dev/A9-QaNLhbZ4/MKMGv7ztHLsJ

refs #1129
Add helper class BigBlueButtonConfigXml

To help users to modify config.xml files.
First version, still needs more documentation and tests.

refs #1129