Amazon S3 for assets storage

• [changed] extract set_position method to concern - used in kms as well as in some extensions
• [added] add FunctionsRegistry service - for extending with Liquor functions
• [changed] remove special edit button for pages, make page title clickable
• [changed] make navigating to edit templates, snippets and assets simpler - just by clicking on names
• [added] add ability to setup Amazon S3 for assets storage
• [fixed] fix pages ordering issue - remove unnecessary request to pages - closes #14
• [added] add more help - about scopes/collection methods
• [changed] Angular 1.4 to 1.5 migration + angular-ui-tree update