Add a trust-level parameter to configure the special remote

[adswa]

This add a new parameter --trust-level to add-sibling-dataverse. Setting this parameter allows setting the trust level of the special remote (otherwise, git-annex's default is used when setting it up).

It might be a fix for #179; it mimics how create-sibling-ria does it (though performs the trust setting via config variable instead of using e.g., git annex trust, as I found its easier to streamline this way and doesn't require --force)

[mih]

Thx for this. I am wondering now, whether this is equivalent in terms of outcome to running git annex trust (or similar).

[mih]

I decided to leave this here, and not put it into the 1.0 release. For reasons of timing and uncertainty re side-effect alone. Thx for implementing it, and sorry for not merging it now.