FragmentPagerAdapter subclass must implement getPageTitle

In `TabBarView` you are calling `pager.getAdapter().getPageTitle(i).toString()` which will throw an NPE if it is unimplemented. Since `getPageTitle` is not abstract, the `CharSequence` returned should be checked.

https://github.com/Mirkoddd/TabBarView/blob/master/TabBarViewLibrary/src/com/mirko/tbv/TabBarView.java#L174
