KML22 - dump feature collections

Following this thread on the ML from last week:

http://sourceforge.net/p/geotools/mailman/message/33052373/

This commit aims to add the write support of feature collections into KML2.2.

Manual tests, and a new test method has been added to check that the produced document is correct.

Note: Some other implementations around expect that the XML produced is not prefixed, and that the default namespace is the KML2.2 one. For example, the document generated in the added unit test won't be detected as valid with my current version of gdal/ogr, and won't work onto http://display-kml.appspot.com/ either. Removing the 'kml:' prefix on each elements and setting it as the default xml namespace fixed the "issue" (even though this is valid IMHO to prefix the whole document).

Feedbacks welcome (ping @jdeolive ;-))