STY : minor style changes

  - no need to clean up query, pretty sure gc will do the job for us
  - pass-through kwargs should go at the end unless you need to preserve
    back compatibility
BLD : moved imports around

 - metadataStore now only imported in __main__
MNT : changed how key related defaults are handled

 - made `keys` mandatory for construction
 - made `key_description` a defaultdict which yields empty strings
BUG : added missing dict to add_btn_sig emit

 - the signal expect 3 dicts
Merge pull request #1 from tacaswell/ericdill-vistrails_integration

Ericdill vistrails integration