Pre-release v1.5 fixes

[tobixen]

Including some real regression bugfixes.

Refers to #491, but sneaking in some other changes to the test framework and compatibility matrix.

[tobixen]

I should look through the CHANGELOG also before committing. And we should yank out niquests, that's slated for v2.0 rather than v1.5

[tobixen]

This got bigger than expected and should have been split into several pull requests - but I don't want to spend time on micro-management.

• Preparations for releasing v1.5.0 - this includes CHANGELOG-entries
• Acceptiong some different behaviour from different servers with obj.load when the object is gone, missing or if the server refuses to give any data on the object.
• Calendar.calendar_multiget and obj.load_by_multiget has been refactored.
• Tests and compatibility matrixes has been altered to allow the tests to pass on all the test servers I have access for.
• Regression fix for an earlier applied workaround (relevant for GMX server at least)
• Downgraded path handling problem error to warning
• Minor cleanups of code and comments