ME: added id_string property that returns id_string in lowercase in XForm model

Hey @pld, 
I am working on this issue (#79) and i consulted @ukanga about my implementation approach.
He said that he is not sure about the other references of 'id_string' in the other sections of the 
codebase based on my approach and asked me to make this pull request so that you can review
it and give me feedback. His approach would be to change the id_string to lower case before we
save. My implementation involves converting the id_string to lower case at the time of retrieval
so that the existing 'id_string's with uppercases can also be converted