Make the client work with all supported Nuxeo versions

See how we can publish only one version of the client that works with Nuxeo LTS 2015, Nuxeo LTS 2016 and Nuxeo 9.1+.

The goal is to ease the maintenance, the install and the use of the Nuxeo client regardless of the Nuxeo server version.

What's needed to be done:

• Detect the Nuxeo server version, force the use of Nuxeo#login to retrieve that information. The client will still work even if the login method is not called, but no check will be done and thus some calls will not work as expected.
• Expose that version to the users of the client so that they can make use of it.
• Define how to list functions/features working or non-working on a given Nuxeo version
• Define what the client should do/return in case of a call that won't work with the target Nuxeo version (such as using Document#fetchRenditions on a Nuxeo LTS 2015).
• Launch the tests on all support Nuxeo versions