Caching last weather location to display weather conditions quicker

When the app is closed then restarted, cached weather conditions are displayed giving the illusion that the app runs faster.

Also added current location button in the action bar.
Creating a base weather fragment class

I noticed some functionality that ApplicationStartupFragment and
DisplayWeatherFragment both implemented, so it's better to have them
inherit from a common base class. Also this is useful when I want to hide/display th
fragments' view.