GWTII-190: Added client-side OSM layer to TMS and sample.

GWTII-190: Added client-side OSM layer to TMS and sample.
GWTII-190: Fixed bug where regular TMS would not render.
GWTII-190: Added newline at the end of file.
GWTII-190: Javadoc and checkstyle.
GWTII-190: Added OSM plugin.
GWTII-190: Updated poms for OSM plugin.
GWTII-190: Added OSM plugin documentation.
GWTII-190: Added OSM plugin to showcase.
should y always be inverted (for every TileBasedLayer?) or is this too OSM-specific?