Fix install of `newrelic_plugin_agent[pgbouncer]` python package

On Ubuntu 12.04 (and potentially others), trying to install
`newrelic_plugin_agent[pgbouncer]` breaks b/c it cannot find
`pg_config`.

Turns out either `libpq-dev` (for Debian/Ubuntu systems) or
`postgresql-devel` (others) is needed for this to succeed. See
http://stackoverflow.com/a/12037133/133479 for details.
install dependencies only when required

this moves installation the following dependencies to the newrelic_ng_plugin_agent lwrp
- newrelic_plugin_agent[mongodb]
- newrelic_plugin_agent[pgbouncer]
- newrelic_plugin_agent[postgresql]
- libpq-dev resp. postgresql-devel
ruby cookbook ref not existent anymore
add test for installing plugin agent dependencies
Revert "ruby cookbook ref not existent anymore"

This reverts commit c8f6048e9914e31412ea065a2c7a5bc83ca2e96d.
fix plugin-agent test attributes
fix hardcoded pidfile path in debian init script
Use `service` instead of running script explicitly

Avoids any sort of environment settings from polluting the service.

See http://askubuntu.com/a/47664 for details.
Add required keys to postgresql stanza in plugin-agent test

Undocumented, but these four keys are required for a postgresql stanza
add contributors to readme
Merge pull request #18 from chr4-cookbooks/install-dependencies-when-needed

Install dependencies when needed