Provide membership counting operations for the directories.

Some directories should allow very fast counting of elements inside them. Crowd should make this functionality avaliable via its api.

For example, in the basic case you can get every membership in a directory and count it manually in code. However, for some directories you should be able to directly ask the question to the directory and get the answer back in much less time. We should revert to the naive implimentation in the common case and speed up counting operations where we can.