Support Roster Versioning from RFC6121

[lnjX]

https://tools.ietf.org/html/rfc6121#section-2.6 (the standard is almost the same as in the obsolete XEP-0237)

This would allow us to avoid fetching the full roster on every connect which is especially good for mobile connections.

Things to be done for this:

• Cache roster stanza with roster version in a file (should be configurable if to do so and where to save)
• on connect check stream feature and include roster version in request (if cached)

The version attribute in the RosterIq is already implemented (5be0f37) (the commit description there is misleading: as an application it's currently not possible to actually use the XEP, because you can't adapt the roster request and can't access the stream features).

This would also fix this issue: #74