Merge branch 'dev'

* dev: (69 commits)
  added data model image to API reference
  deply only a single master build
  djusted .travis.yml
  adjusted .travis.yml
  missing final
  datamodel image added
  authentication header adjustments
  user.create for jekyll added
  (empty) user.create.json
  group description
  measurement -> measurement.create
  fixed typo
  only one schema update
  groups.md updated for jekyll
  Added users example and formating of measurements, tracks and friends.
  rewritten api/index
  sensor jekyll things added
  further merges
  restructured users.md
  user modify example added for jekyll
  ...
Merge branch 'dev'

* dev:
  use local schemas
Merge branch 'dev'

* dev:
  added documentation readme
badges support (fixes #114)
only show activities of bidirectional friends (fixes #95)