Minor test case additions & lxml warning

- Added a few tests to show what kml is produced by adding a style to both a kml.Document and kml.Placemark. Also expose a few different ways to generate the same desired kml output string.

- Added logger in fastkml.config which logs a warning if lxml is missing. This indicates that the pretty printing will not work (no newline / '\n' characters will be inserted). This was a tricky thing that took me a while to figure out why the newline characters were not being added to the output kml string.