Fix stack-overflow when editing Doctors clinic to be the same.
Add Clinic model before_destroy callback.

  * Reset all foreign keys of doctors with the record's id to NULL.
  * Fixes issue #6
Add :dependent => :nullify to Clinic.doctors association.

  * When you delete a clinic, will nullify the relevent clinic_ids for
    each Doctor with the FK.
  * fixes issue 6
Allow Doctors to be added and removed from Clinic form.

  * fixes issue #8
  * Add and remove doctors from clinic/:id/edit
  * Show associated doctors in clinic/:id

  * Add/remove logic is in the model, the update controller determines
    which models need to be adjust based on the form POST data, then
    hands it off to the model to the adjustment.
Revert association validation from previous commit.

  * It was only the circular reference causing the overflow.
Merge branch 'hotfix.recursion' into devel
Quick edit to colspan in doctors.index view
Minor whitespace and comment fixes.
Remove useless variable.
Merged in tchamberland-wip/clinic-app (pull request #2)

Bugfixes and minor interface changes.