Content Handler: Add support for custom attributes
Content Handler: Add filter hook for adding custom attributes

* Hook: `slb_content_handler_[handler-id]_attributes`
Content Handlers: Load custom attributes into client
Merge branch 'chdl-attrs' into develop
Add Grunt support

* Gruntfile.js
* package.json.js
Merge branch 'grunt-init' into develop
Move JS files to subdirectory (for automation)
JS: Validation via jslint
Optimize: Remove deprecated JS methods in code

* `escape()`
* `unescape()`
JS: Optimize whitespace
JS: Optimize line breaks
JS: Optimize variable declarations
JS: Add missing semicolons
JS: Remove duplicate `Viewer.unload()` declaration
JS: Optimize `switch` statement
JS: Remove unused variables
JS: Optimize conditional statements (equality/inequality)
JS: Optimize core class definition

* Move method handler to discrete function
* Remove references to `arguments.callee` (deprecated)
Update references to JS files (use new location)
Grunt: Update JSHint options (newcap)