Adding challenges users model
Merge in master; fix conflicts with Gemfile
Merge branch 'development' into jr/challenge_participants
Merge pull request #68 from ijustwanttocode/jr/challenge_participants

[challenges] Add challenge participants
Merging in dev; fixing Gemfile conflicts
Adding specs and some cleanup
Merge pull request #60 from ijustwanttocode/jr/add_withings_scale_measurement_service

Add withings scale measurement service
Merged development
Added additional test for measurement service