Before, it was the intention to do something like this:

```python
m = Modules()
m.add_module(['GCC/4.7.2', 'gzip/1.5'])
m.load()
```

This is kind of stupid, because if you want to load a different set of modules, you need to create a new instance of `Modules`.

With this changes, `add_module` should not be used anymore, and `load([...])` should be used instead, thus a `Modules` instance can be reused.