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.
I don't want to implement this twice, though, even if you personally would only use it for EULA-type things. I can see some admins wanting to add a warning to say, a Yosemite upgrade: 

>**Important Yosemite Upgrade Information**

>This upgrade will take at least an hour. You should leave your Mac connected to a power source. The following software in use by Foo Company is known to have issues with Yosemite: 
>Bar
>Baz

>Are you sure you want to update to OS X Yosemite?

>(Cancel)  (Upgrade)