Clean up the tabs directive

[ajoslin]

It's gone through many changes since Ionic started, so a fresh look at how it works and why look would be a good idea.

Below related issues in one list:

• tab hidden/shown event should give index of tab hidden/shown #634: tabsController is on <ion-tabs> scope, user can watch selectedTab or getTabIndex(selectedTab)
• has-tabs refactor #620, Feature: Ability to hide tab bar #395, added support via view directive to hide tab bar #419 has-{tabs,header,footer,etc} css/js refactor
• Tab title #334 fixed
• Tabs Directive: allow to bind the active tab #175 Use tabsController.select - it takes an index or a tab scope
• ion-tab directive should support scope variable binding #646 fixed

wontfix:

• ion-directives could broadcast gestures to controller #647 cannot see a better way to do this one than what the user is doing, unless we move to element nav-items (which could be done later). Example: <ion-tab><ion-tab-nav on-swipe-left="previousTab()"></ion-tab-nav></ion-tab>

[adamdbradley]

Related: #634 #620 #334 #175 #646 #395

[ajoslin]

#419, #647