Add a customer relation preference for communication language.
New ERP5 SyncML implementation

Complete rewrite of the syncml core engine to make it scallable
Remaining work :
- review of conflict management
- re-implement splitting of big object
- generic unit test for AsynchronousEngine
- complete review of default conduit
- lot of TODO and XXX in the code to check
Merge remote branch 'origin/master' into security_groups