Refactor plugins to allow for ordering with #:before / #:after

Example: (register-pre-all-plugin 'plugin-name plugin-function #:before 'some-other-name)
  That will run some-other-name before plugin-name

If there is no way to satisfy the ordering, you'll get an error
Added two new plugins: atom and photosynth

- atom is the inital work to generate feeds, untested
- photosynth takes an ID and generates the necessary iframe to display it