Attempt to sort based on primary key in scope methods on VersionConcern if primary key is an integer.

This makes some of the changes to these scope methods that came from 6a4aba2 more flexible,
in that the user can choose to compare timestamps if desired, but it defaults to comparing and sorting,
via the primary key (if it is an integer).  If the primary key is not an integer, it still defaults
to using the PaperTrail.timestamp_field.

This is my proposed fix for #314, and I also believe it should fix #317. It seems that that this issue
is usually encountered when testing PaperTrail with MySQL (presumably due to lack of microsecond timestamp support).