md-tabs: add refactored tabs code

[robertmesserle]

Key improvements:

• Consolidate code into a single manageable controller
• Reduce the amount of code contained inside directives
• Remove communication between directives and controllers via $broadcast
• Rely more heavily on data-bindings and do less DOM manipulation manually
• Consolidate all tab logic to a single watcher on selectedIndex
• Remove performance hit of repeatedly restyling the tabs to calculate ink bar position by using a hidden copy of the tabs for all calculations
• Remove complex logic required to handle pagination on keyboard tabbing by using hidden tabs for keyboard interaction (ie. pagination can happen immediately on focus, rather than waiting for pagination to complete before allowing focus)