Tab: active state

[RobJacobs]

@wesleycho I'm seeing a behavior I was not expecting as a result of change #5425 and wanted to check if it was intended. The tab content no longer works if not implementing the 'active' state on the uib-tabset and 'index' on the uib-tab directives. Previously, tracking the active tab was not required. Here is a plunk demoing the issue.

[wesleycho]

The API changed as a result of 5ec19b6