Copyright update.
Initial commit.

The plugin was first named json_decoding_plugin but changed the name
after discussion on -hackers. It is a POC to illustrate that it is
possible for changeset extraction work just fine. The changeset
extraction feature is already in development, hence, it is expected that
this code changes a lot until the feature is set in stone.

It is based on:

repository: git://git.postgresql.org/git/users/andresfreund/postgres.git
branch: xlog-decoding-rebasing-remapping
commit: 6f819a68cd10938df126ee44a3cfef7f457c8e1b