Add optional preinstall_alert key to pkginfo plist.

Displays text that end users can agree or not before continuing with install.

This is a requirement for some software, such as skype or dropbox, that may be provided but needs extra conditions outside regular acceptable use policy that the user needs to be aware of e.g. a user must agree that while dropbox is permitted, to not store protected IP in a dropbox folder.

The added key:string pair user_agreement will contain the content of the message on a per package basis.
Wouldn't this all be cleaner/more readable as
```python
        if item['status'] == 'not-installed' and item.get('preinstall_alert'):
            self.clickedItem = item_name
            self.displayPreinstallAlert_(item['preinstall_alert'])
        else:
            self.actionButtonPerformAction_(item_name)
```