Observe adapter dataset and notify View of changes

If the adapter's dataset changes, `PagerSlidingTabStrip` does not apply the changes before `onPageScrolled` is called. This causes an NPE because `tabsContainer` has no children.

With this fix, `PagerSlidingTabStrip` is now synced with its `ViewPager`'s adapter (the correct tabs are present) and `tabsContainer#getChildAt` will no longer throw an NPE.

Fixes #66