Close #11: Fix broken HTTP DELETE verb

* common.lisp (destructive-p): DELETE is also a destructive verb.
(handle-request-1): DELETE doesn't care about content-types

* snooze-tests.lisp (:snooze-tests): import some symbols from
snooze-tests-demo.
(:snooze-tests-demo): Move package definition to top.
(todo): Allow making TODO with given id.
(with-request): Bind SNOOZE:*CATCH-ERRORS* and SNOOZE:*CATCH-HTTP-CONDITIONS*
(todo :delete): New route
(test-delete-route): New test
(*mock-http-payload*): Move symbol to :SNOOZE-TESTS-DEMO