better string representation for inclusive datespans, add offset param to since fn
refactor based on PR feedback
Merge branch 'datespans' of github.com:dimagi/dimagi-utils into datespans
Merge branch 'master' of github.com:dimagi/dimagi-utils into datespans
Merge pull request #120 from dimagi/datespans

better string representation for inclusive datespans, add offset param t...