update Crescent Lake

Velensk posted a new version here:
http://laddercouncil.proboards.com/thread/23/crescent-lake?page=2
add 4p Crescent Lake, add it to a new "Explorer 2v2" map pool

Explorer 2v2 extends Classic 2v2
add Ruins of Terra Dwelve to the Explorer 2v2 set
add Underworld to the Explorer 2v2 set
Merge branch 'master' of git://github.com/WesnothRBY/picker