added a StringField to be used in IndicatorDocuments to perform simple operations such as extracting values from a case
added flat field that provides a callable that accepts a case and returns a string
Merge branch 'master' into flatfield
Merge branch 'prepend_doc_type_to_key' of https://github.com/dimagi/fluff into flatfield
Merge branch 'allow_date_range' of https://github.com/dimagi/fluff into flatfield
Merge branch 'master' into flatfield
Updated docstring
code clean-up from PR comments
Merge pull request #57 from dimagi/flatfield

Flatfield