Define Module

A proposition for #27

Restrictions:
- All users can use it.
- 15000 calls per hours.

Dependencies:
- Require an API key from http://developer.wordnik.com/ (free and instant delivery).

Usage:
- !define pie

Actions:
- Get a word definition from internet.

Intern Features:
- Recursive call to "!define {word}" will iterate all definitions then restart form the first one.
- Words are keep in cache for 2 minutes then deleted to to avoid unnecessary memory occupation.
Update example.config.js

- Add a property for wordnik api key.
Merge pull request #49 from Pawda/patch-3

Added define Module