cater for fluff value emitters
combine rows in memory, lazy create extractor
remove has_value
Merge pull request #6 from dimagi/fluff_emit_value

cater for fluff value emitters
fix stale metadata bug
Merge branch 'master' into munge
Merge pull request #9 from dimagi/munge

combine rows in memory, lazy create extractor
Merge branch 'master' into run

Conflicts:
	ctable/base.py
	ctable/tasks.py
	ctable/writer.py