Adding postfix plugin for ohai-solo

I wrote a little plugin for ohai-solo to discover current postfix configuration.
I decided to make it work only in case of RPM/DEB installed packages, and to look only at very few, but important config options, which can tell why mail delivery might not be working or not.

I thought about adding a method to check the size of outgoing mail queue, however I remember that running mailq on a server that has a huge queue, will take ages, before it produces the result you want, therefore I decided not to include that.

Marcin
If no postfix_bin is found, this should not return anything. Otherwise the output is:

```
  "postfix": {
    "postfix_binary": "",
    "postfix_package": null
  },
```

Check out the [find_apache_executable](https://github.com/rackerlabs/ohai-plugins/blob/master/plugins/apache2.rb#L163) for an example.

Probably need to add some checks to the `collect_data` section to handle an empty return from `find_postfix_executable` as well.