Update NuGet-2.8.markdown

fixed the very bottom of the release notes to have 2.8 data instead of 2.7
Update NuGet-2.8.markdown
Update NuGet-2.8.markdown

updated the notes to include:
 - the DependencyVersion Attribute section
 - modification of the -DependencyVersion Switch section to provide the scope in which the feature works.
 - included the major performance improvements in the update-package -reinstall in the Bug Fixes section
Update NuGet-2.8.markdown
Update NuGet-2.8.markdown
Merge pull request #163 from joostdenijs/nuget-2.8-release-notes

pull request reviewed by Howard. Merge it now.