[PATCH] Implement svnadmin verify --keep-going

Hi,

This patch is a follow up of the long discussion we had in thread [1].
This patch implements a new switch "--keep-going" to svnadmin verify.

If "--keep-going" is set(True), svnadmin verify would continue to run
till verifying all the revisions i.e, it would not stop at the very
first occurance of a corruption/error found in the repo and would report
corruptions wherever found.

I have worked on your suggestions and inputs for this patch. Kindly
share your thoughts. Attaching the patch and the log message with this mail.

Thanks and regards
Prabhu

[1] http://svn.haxx.se/dev/archive-2012-10/0271.shtml