Remove 'tags' special attribute; all attributes are now tags.

- Required change in query syntax: use
  myfile[ {'period':'am','mode':'trn'} ]
Merge remote-tracking branch 'omx/dev' into jraw
WIP: 0.2 changes
Merge branch '0.2' into dev: new file structure, new tag format

- tags are now just regular hdf5 attributes
- implement /data and /lookup top-level folders
Fix key/value attribute lookup
fix bug in __setitem__() which was copying matrix to root instead of /data
Merge remote-tracking branch 'omx/dev' into jraw