Using Android recommended storage directory

Issue #275 

This fix uses `getExternalFilesDir` to ask for the application folder.  It will no longer assume that `/sdcard/GPSLogger` is writeable.  

This in turn is because different manufacturers are implementing this restriction in different ways. 

It's also [really, really confusing](http://commonsware.com/blog/2014/04/09/storage-situation-removable-storage.html).