UK: get id_string from data/instance
UK: use pyxform branch with xform2json updates, allows most xforms to be uploaded by briefcase
UK: pin down jsonfield to version lass than 1.0
Merge pull request #528 from onaio/pyxform-briefcase-upload-issues

allow briefcase to upload xforms and submissions
ME: added id_string property that returns id_string in lowercase in XForm model
ME: added lower case conversion of form id_string
ME: added a checker; checks if id_string is converted to lower case when form is being saved