pg_rewind in contrib

Hi,

I'd like to include pg_rewind in contrib. I originally wrote it as an
external project so that I could quickly get it working with the
existing versions, and because I didn't feel it was quite ready for
production use yet. Now, with the WAL format changes in master, it is a
lot more maintainable than before. Many bugs have been fixed since the
first prototypes, and I think it's fairly robust now.

I propose that we include pg_rewind in contrib/ now. Attached is a patch
for that. It just includes the latest sources from the current pg_rewind
repository at https://github.com/vmware/pg_rewind. It is released under
the PostgreSQL license.

For those who are not familiar with pg_rewind, it's a tool that allows
repurposing an old master server as a new standby server, after
promotion, even if the old master was not shut down cleanly. That's a
very often requested feature.

- Heikki