use plain javascript date and simple input field for date attributes

+ remove bootstrap-datepicker-rails
update changelog
remove bootstrap-datepicker-rails from test app
allow to clear date values

fixes #294
support enter and esc keys
Merge branch 'feature/date-editor' into develop

* feature/date-editor:
  support enter and esc keys
  allow to clear date values
  remove bootstrap-datepicker-rails from test app
  update changelog
  use plain javascript date and simple input field for date attributes