Rails: Add tag / taggings model / migrations + associations.

Also, add some inverse_of associations across app.
Backbone: Add tag model / collection. Rails: edit tag / task as_json.
Rails: Add tags controller. Minor edit to rails controller...

Bootstrap in tags.
Rails: tags now belong to users.
Backbone: Adds basic tags index view, templates, routes.
Rails: Add seed data for tags.
Backbone: fix bug preventing tags from loading.
Backbone: add tags link to menu.
Styling: more generic styling for app-content-header.
Backbone: fix broken tag link.
Style: style tag index page.
Backbone: add tag form - implement tag editing / deletion.
Styling: Refine tag css.