Added support for `LevelDB` to Current!

[dkorolev]

This works now: https://github.com/dkorolev/current_with_leveldb.

mkdir -p clean/with/subdirs
(cd clean/with/subdirs; git clone https://github.com/dkorolev/current_with_leveldb) &&
(cd clean/with/subdirs/current_with_leveldb; CURRENT_WITH_LEVELDB=true make) && (
  cd clean/with/subdirs/current_with_leveldb &&
  ./.current/create_leveldb &&
  ./.current/scan_leveldb &&
  ./.current/populate_leveldb &&
  ./.current/scan_leveldb &&
  ./.current/populate_leveldb --key foo --value bar &&
  ./.current/scan_leveldb
)
rm -rf clean/with/subdirs/current_with_leveldb
rmdir clean/with/subdirs
rmdir clean/with
rmdir clean

I've also pushed leveldb into https://github.com/c5t/leveldb.

Thanks, Dima

[dkorolev]

@dkorolev: Note to self, clean this up to use leveldb as a C5T component. CC @mzhurovich.