Fix severe bug in davstorage

where (href, etag) instead of etag would be returned.

Thanks for @slavkoja for finding it.
Add GuessAuth to authentication methods

Also fix a bug where digest auth would always be interpreted as basic
auth. Jesus, does anybody even use anything else than basic auth?

See #33
Fix radicale auth
Greatly simplify monkeypatching code for radicale
Add requests_toolbelt to requirements
Remove nonsensical sentence
Move requests_toolbelt to real dependencies