Specify anchor locations for the UIActivityViewController

This addresses issue #45 in which an exception would be thrown when using the extension on iPads due to the lack of a presentation anchor for the share sheet popover. Either (sourceView, sourceRect) or (barButtonItem) must be specified on iPad (on any device which presents the sheet as a popover) to prevent this crash.

It may also be necessary for some users to specify the popover arrow direction, but that is not in this changeset.