Add fallback to main_app's logout_path if not using Devise.
Move logout_method to application helper and add fallback to delete if Devise is no present (need a Config for this).
Merge pull request #2062 from jvortmann/fix_logout_path_and_method

Fix #1386 by removing dependency of Devise for logout_path and logout_method.
Pass symbol as an argument instead of a block