0.2.0

CHANGES:

• unikernel: upgrade to mirage 4.3 (#31 @hannesm)
• bugfix: href_to_principal always returned "/principals" (since v0.1.1)
  (#24 @hannesm)
• error when principal or calendar already exists when adding a new principal
  (#24 @hannesm)
• when modifying an ACL, check that all referenced principals exist
  (#24 @hannesm)
• check for groups and principals on operations thereof (i.e. disallow deleting
  a group when the principal is a user, etc.) and test cases (#25 @hannesm)
• use batch in Webdav_api instead of Webdav_fs to reduce commits
  (#26, fixes in #27 @hannesm)
• bugfix: adding and removing groups and users, only edit prop file once
  (#27 @hannesm)
• bugfix: compute etag after batch (#27 @hannesm)