Add param for specifying validate connection script in postgresql::client.

* Add param `validcon_path` in `postgresql::client` (defaults to
previous hard coded value).
* Add tests for this new parameter.

All tests runs successfully on Scientific Linux 6.4

```
$ bundle exec rake spec SPEC_OPTS='--format documentation'
[...]

Finished in 1 minute 58.61 seconds
159 examples, 0 failures
```

Travis passes all tests except for `PUPPET_GEM_VERSION="~> 3.5.0.rc3"` for an external reason:
```
/home/travis/.rvm/gems/ree-1.8.7-2012.02/gems/json-1.8.1/lib/json/ext/parser.so: [BUG] Segmentation fault
```