Merge pull request #18 from ajbeamon/master

ExistsAsync uses the just computed directoryLayer instead of this.DirectoryLayer
Merge pull request #19 from ajbeamon/master

Some fixes to NodeContainingKey
Tweaked SliceWriter.Skip(...)
Fixed bug where FdbSubspace.UnpackSingle<T>(...) was calling FdbTuple.UnpackLast<T>(...)