Add a LevelDB API to WiredTiger

This branch adds a LevelDB API, with flavors for some forks (Basho's LevelDB, HyperLevelDB and RocksDB).
@agorrod thanks for catching this.  I agree it's unrelated to LevelDB, but I think there is a real issue here.  I'll back this out of the branch, merge, then open a real issue for this one.