update gems, add `guard`
update rubocop config
add Guardfile
support "mixed-mode" configuration:

* previously, if `node["postgresql"]["conf"]` was
  set, a custom-only configuration file was used.
  now, the default template will be used and will
  append any options in the `conf` hash, which
  makes it easier to extend the configuration
  without uncommenting all of the existing values.
* if `node["postgresql"]["conf_custom"]` is true,
  the default template will not be used and only
  values found in the `conf` hash will be written
  to the configuration.
Merge pull request #100 from phlipper/mixed-config

Support "mixed-mode" configuration

closes #68