Rename MemorySource#_findFirst -> _filterOne

"find" methods should return promises
Comment cleanup
sendClientIds option allows client ids to be sent to the server

Otherwise, they're only sent if they match the remoteId
Publicize methods: ajax, buildURL, pathForType, pluralize, serialize, deserialize
Remove JSONAPISource#sendClientIds option
Simplify _findLink
Reverse links are now stored in `__rev` instead of `refs`
Client-side links are now cached as `__rel` and `__rev`