Added reference content for package restore and TeamBuild

As discussed with @jeffhandley, the purpose of this pull request is adding documentation around how one can enable package restore in Team Build.

Originally, Jeff proposed to add the content directly to `site/Docs/Reference/Package-Restore` but I felt this would make this topic a bit unwieldy. That's why only added a section there and put the actual content in separate topic.

To summarize my changes:

- Added section for wrapping the new package restore in an MSBuild project
- Added new topic with a walkthrough that shows how to setup Team Build and package restore
- Added an entry for the new topic under `site/Docs/Reference/_metadata`. Not sure whether that's required, though.

Feedback welcome!