resolves #201 add bundle as subcategory of ui category in playbook schema

• move ui.bundle key to ui.bundle.url
• move ui.start_path key to ui.bundle.start_path
• introduce ui.bundle.snapshot key to provide hint that URL is a snapshot
• add interim logic to migrate keys from old location
• update tests and API docs
• rename --ui-bundle arg to --ui-bundle-url
• document that ui.bundle is now ui.bundle.url
• document that ui.start_path is now ui.bundle.start_path
• document ui.bundle.snapshot key
• fix indentation in playbook file excerpts (2 spaces instead of 1)
• correct explanation for how and where UI bundle is cached
• move rules for expanding relative path below example