PLD: check that field exists when asking for field in states, closes #148
Merge pull request #221 from onaio/check-field-existence-148

PLD: check that field exists when asking for field in states, closes #14...
UK: test different users can have and access same id_string form
UK: get_xform by id_string, ensure username/owner is supplied, closes #174
PLD: pep8 cleanup