Added optional user_agreement key. Displays text that end users can agre...

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.
That makes sense. So it's some important alert to display then.

install_alert ?
<dict>
  <alert_title>
  <alert_text>
  <ok_alt_label>
  <cancel_alt_label> 

I can't imagine a need for more than OK/Cancel since it's not a complex decision path (a Maybe?...what would it do?). Default values for OK, Cancel if not specified (and Title?..or no title if not specified)