Use pg_regress for make installcheck testing

This uses pg_regress to start a memcached instance, run test queries
from test.sql, and stops the memcached again. Needs root.

apt.postgresql.org uses this for integration testing for the Debian
packages; these tests discovered the incompatibility with libmemcached
1.0.16.
Merge pull request #24 from ChristophBerg/regressiontests

Use pg_regress for make installcheck testing