Added get_keys() method to get subsection keys.

The subsection and subsection_keys (aka subsection_list) members
are now private.

For instance, if myconfig.mysection contains __many__ and so has
subsections, in order to iterate (in order) over all subsections:

```
const subsection_keys_type & keys = myconfig.mysection.get_keys();
for(subsection_keys_type::const_iterator k = keys.begin();
    k != keys.end();
    ++k)
{
    cout << "subsection name is " << *k
         << " parameter is " << myconfig.mysection.at(*k).parameter
         << std::endl;
}
```

The subsection_keys_type is an alias to std::vector<str_type>.

@zzotta: you must now change your code!

Closes #98