PermisoDialogFragment should extend android.support.v4.app.DialogFragment, not android.app.DialogFragment

[paulpv]

Admittedly, in 2016, the number of Android devices that don't support android.app.DialogFragment is getting very small, but to increase backwards compatibility PermisoDialogFragment should extend android.support.v4.app.DialogFragment instead of android.app.DialogFragment.

This would require changing "WeakReference mActivity" to either "WeakReference mActivity" or "WeakReference mActivity", which results in a few other required changes in the code.

Would you consider doing this, or a PR?