Merge tag '0.6.3' into develop

Vist 0.6.3 upon the world

 - Implement the web backend
 - Bugfixes to the fs backend
 - Document everything
README improvements
Document api/list-artifacts
Document api/list-groups
Match docstring style with api/list-groups
Use Failure not Exception
Document api/list-versions
Document api/list-namespaces
Document api/list-classes
Document api/list-defs
Document api/thing->prior-versions
Note nested Failure case
Document api/read-notes
Document api/read-examples
Document api/read-meta
Add a note about the API's assumptions
Add a warning about possible future work
Document api/read-related
Remove empty ns
grimoire.api.fs -> grimoire.api.fs.impl