Added optional Close button

Hi Adam,

This gives the client the option to show a "Close" button at the bottom of the licenses page. It maintains backwards-compatibility so you can still use the old creation methods to display the dialog fragment.

I wanted to give end-users who aren't that familiar with Android a simple way to close the dialog in portrait mode instead of them accidentally hitting the Home button and exiting the app. I think it also gives it a nice professional look, even though the dialog looks very nice as it is :)

I used a standard method of displaying buttons on a DialogFragment by having it return an AlertDialog from `onCreateDialog()`, where the AlertDialog's content is set to `licenses_fragment.xml`.

Try it out and let me know what you think.