feat(tab): add bsActivePane attribute to bind active tab pane

Currently the tab directive is using ngModel to bind the active pane index. This causes the tab control to be included in the validation pipeline and when used inside a form, causes the form to be $dirty. (see issue #1233)

This commit deprecates usage of ngModel and adds a replacement bsActivePane attribute with the same functionality, so it should be a matter of replacing ngModel references with bsActivePane.
docs(tab): add bsActivePane helper attribute description