Clear back stack on tab change.

Otherwise back stack retains history for all tabs together, which can result
in inconsistent state and unexpected behavior.  See discussion here:
http://stackoverflow.com/questions/6987334/separate-back-stack-for-each-tab-in-android-using-fragments
Merge pull request #49 from flibbertigibbet/feature/clear-tab-back-stack

Clear back stack on tab change.