sync: starting with am interface for syncing changes between objects stored in
a local or remote data store. The interface for now includes:

## read
- `#get`
- `#subscribe`

## write
- `#set`
- `#commit` (transaction semantics)

Thinks I'm working on:
- (sub)handle access
- reading tree changes
- synchronization for collections
sync: adding tests to run_tests.js
sync: cleaning stuff
sync: child handles
sync: child handles are updated when parent changes
(internally now using an objectified path representation)
sync: fixing model lookup bug
sync: fixing ObjectHandle>>get
sync: commit interface uses options obj
sync: handle.off fix
sync: refined test
sync: adding model-sync world for now
sync: handles using object-based parameters consistently
sync: off -> unsubscribe
model-sync: fixing get
model-sync: cleaning up callback logic
model-sync: subscribe to child changes
model-sync: switching to lively.PropertyPath
model-sync: consistent lively.PropertyPath usage
model-sync: adding async local store for simulating distributed environment
model-sync: fixing callback path