Build xml documentation for NuGet.Core

[hickford]

Fixes NuGet/Home#2381

[hickford]

Did I choose the right target branch? I would have picked master but the most recent commit was May 2015

https://github.com/NuGet/NuGet2/branches

[dnfclas]

@hickford, Thanks for signing the contribution license agreement so quickly! Actual humans will now validate the agreement and then evaluate the PR.

Thanks, DNFBOT;