Allow custom metadata to be attached to panel/df/series?
related:
https://github.com/pydata/pandas/issues/39 (column descriptions)
https://github.com/pydata/pandas/issues/686 (serialization concerns)
https://github.com/pydata/pandas/issues/447#issuecomment-11152782 (Feature request, implementation variant)

Ideas and issues:
- is pickle stable enough?
- serialization req. potentially couples this with storage format
- allow arbitrary python objects?
- impl. option: JSONables-only + type tag + load/save callback hook
- getter/setter or attribute style access (limits naming)?
- flat or hirarchical?
