Supports CREATE EXTENSION.

I've changed lib/Makefile to support CREATE EXTENSION.
In order to work with previeous PostgreSQL version (i.e. < 9.1),
this implementation installs both pg_reorg--1.0.sql and pg_reorg.sql.

This patch lacks regression testcases for CREATE EXTENSION, which I will commit soon.

lib/pg_reorg.control

@@ -0,0 +1,6 @@
+# pg_reorg extension
+comment = 're-organizes a PostgreSQL database'
+default_version = '1.0'
+module_pathname = '$libdir/pg_reorg'
+relocatable = false
+schema = reorg