Include GitHub release notes in generated NuGet packages

[lholman]

Leaning on something I previously implemented for a different library over here

There's currently no easy way for people to know what they're getting in a new release of the OneBuild nuget package.

Regardless of the source of the release notes we require them auto populated in to the nuspec releaseNotes element during the Invoke-Commit phase of OneBuild so they appear on nuget.org as well as in the .nuspec file, like so.

[lholman]

It goes without saying that this feature should be available for the solutions that OneBuild is building.

[lholman]

Slightly de-scoping here. This implementation will add the following tasks to OneBuild.build.ps1 which will be be run as dependencies of the New-Packages task.

1. Get-GitHubRelease - To retrieve the highest numbered matching Release from Github for the current [major].[minor] build number found in VersionNumber.xml. Conditionally runs only if a value is provided for the -githubApiKey parameter
2. Update-ReleaseNotesFromGithub - Updates the empty '<releaseNotes></releaseNotes>' element within all .nuspec file(s), if the empty element does not exist the release notes will not be added.

Out of scope: Any automatic updating of or publishing of the GitHub release via the GitHub API.