sort() and limit() for MongoProvider
_limitedDiffFromVersion computes diff for limited and sorted collection
- added skip() to complement limit() in MongoProvider
- fixed concurrency issues when modifying data while getting diff
- enhanced example client (data changes are now synced in UI as well)
example client timeout set to 100ms
fixed issues with concurret modification of same field by more clients
Merge branch 'master' into limit_and_sort_support

Conflicts:
	example/example_server.dart
	lib/src/mongo_provider.dart
	lib/src/subscription.dart
Fixing tests.
Merge pull request #17 from cleandart/limit_and_sort_support

Limit and sort support