Bug-115: add MockNetworkUtils to tell WordsStorage we have nework.
Bug-80: enable actionbar's back button
Merge pull request #52 from john-hu/bug-80-back-button-actionbar

Bug-80: enable actionbar's back button, r=thomas
Merge pull request #51 from john-hu/bug-115-test-failed-in-real-device

Bug-115: add MockNetworkUtils to tell WordsStorage we have nework., r=thomas
Bug-116 Make MainActivity can select different category.