Add the @since decorator

The idea is to be able to alert the developer that a given API is not available on the server in use.

For instance, if the server version is 9.10, then Comment API is not available. And that code will always fail:

doc.comments()
doc.comment("My new comment")

I the server.log, such errors will be visible:

Service comment not found for object: /nuxeo/site/api/v1 of type APIRoot
Service comment not found for object: /nuxeo/site/api/v1/id/<DOCUID> of type Documen

To prevent that, I propose to create a @since decorator used for each and every method to specifiy the minimum server version:

@since("10.3")
def comment(self, ...):
    pass

@since("10.3")
def comments(self, ...):
    pass

The decorator will have access to the current server version (not yet decided how) and will raise a custom exception.