move camp helper methods into camp model
move camp helper methods for start and end date to model, amend views, delete camp_helper and spec
amend camp_registrations#destroy for current_parent only
use none instead of empty for check on camp_sessions
remove unused strong params from charges controller
use application helper for formatting date
use env var for stripe api key in charges new view
Merge pull request #27 from margOnline/refactor

Changes in response to code review comments