Make error optional in browse-kill-ring-target-overlay-at.

Allow this function to return nil in the case where there's no suitable
overlay at POSITION.
Use -target-overlay-at for -current-string.

Code reuse, there's now a common piece of code that looks for suitable
overlays at a position, lets use it where we can.
Change -current-index to use -target-overlay-at.

Should better handle the case where there are other overlays active in
the browse-kill-ring buffer.
Update -forward to use -target-overlay-at not overlays-at.

This enables the code to handle the case where there are other, non
browse-kill-ring, overlays active in the *Kill Ring* buffer.
Update -edit to use -target-overlay-at not overlays-at.

This enables browse-kill-ring to handle the situation where there are
overlays active in the *Kill Ring* buffer that are not from
browse-kill-ring.
Handle non browse-kill-ring overlays during item highlight.

Use browse-kill-ring-target-overlay-at rather than overlays-at to allow
for non browse-kill-ring overlays that might exist in the *Kill Ring*
buffer.
Update -search-forward to use -target-overlay-at.

Update browse-kill-ring-search-forward to use
browse-kill-ring-target-overlay-at rather than overlays-at so that we
can support non browse-kill-ring overlays existing in the *Kill Ring*
buffer.
Merge pull request #47 from T-J-Teru/linum-fix

Support non browse-kill-ring overlays in the *Kill Ring* buffer.