Add option to skip scanning for POIs in regions

Sometimes, people just use --genpoi for manual POIs. This option
allows them to save some time by not handling entities.
Merge pull request #908 from CounterPillow/genpoi-skipscan

Add option to skip scanning for POIs in regions
overviewer now pulls fire/lava/water textures from the right place

fixes issue #910
Merge branch 'master' of git://github.com/overviewer/Minecraft-Overviewer