Added deleteOsd;
Added updateSysMeta.
fix doc
Added saveContent
- fetchMetadata now expects an id, not an osd object
- added documentation for saveContent
- added updateMetadata()
- added saveMetaset()
Add lock / unlock methods.

Fixes #14
Add RelationType

Closes #12
First version of Relations.

Needs tests, which require more methods (create/remove relation etc)

Closes #13