Celery is disabling pickle serialization and swithing to json serialization.
Json does not have object types.
Current tasks use DateTime object as task arguments.

Converting the tasks so they use iso8601 string instead of DataTime object.

Added tests for calendar and gpx task.

Use colander for form validation in all tasks.

Can't use colander to rename keys, so renamed keys in form.js.
Merge branch 'jsonserializer'