Server compatibility hints

[tobixen]

This is something I have been considering for a long time - I'm a bit surprised that I haven't created an issue on it yet (or perhaps I forgot about it and didn't search well enough).

• DAVClient constructor should have a server_compatibility_hints, probably a dict, probably more or less built over the definitions in place under tests.compabiliiy_issues.
• In some cases, the library should work around known compatibility-problems, for instance filtering searches on the client side.
• In other cases, it should raise an error (perhaps a new ServerCompatibilityError) if it's to be expected that the server won't manage to fulfill a request.
• The current tests.compabiliiy_issues should be retired, and tests should honor the server_compatibility_hints instead.

(See also #401 and others)

I think this should be prioritized, but 1.3.10 or 1.4.0 is overdue, so it won't be dealt with until after the release.