Allow overriding the 'created' timestamp in the docprops.

Can be specified as option to Package#new:
```
Axlsx::Package.new :created_at => time
```

If omitted, the current time at the moment the document is serialized will be used. This change is therefore fully backward compatible.
Merge pull request #195 from die-antwort/allow-override-for-created-at

Allow overriding the 'created' timestamp in the docprops.