Allow attribute filter to be passed on queries, use during group lookups (Fixes: #7)

[charles-dyfis-net]

Avoids downloading full member list during group lookup (an unwieldy use of bandwidth and cache space).

[mcdonc]

This is probably better as:

 attrlist = kw.get('attrlist')
 if attrlist is not None:
     attrlist = tuple(sorted(attrlist))

 cache_key = ( ....
   attrlist
  )

If only because there needn't be a requirement that attrlist be an actual list instance (e.g. if it's already a tuple).

[mcdonc]

Note that the above is probably superseded by the comment below.

[mcdonc]

See comments above.

If you choose to make changes like this, maybe you could also add your name to the file I just added to the master named CONTRIBUTORS.txt within an update to this pull request and also add a change note the the CHANGES.txt file and update the docs in docs/index.rst with whatever changes you have?

[charles-dyfis-net]

Sorry about the delay there -- took a little longer than I intended to get signoff. Should be ready for merge.