source linking NuGet.Core works with SourceLink.Build 0.2.1

Core.csproj requires just two settings to work:
```xml
<SourceLinkRepoUrl>https://raw.github.com/ctaggart/nuget/{0}/%var2%</SourceLinkRepoUrl>
<SourceLink Condition="'$(Configuration)'=='Release'">true</SourceLink>
```
![nuget](https://f.cloud.github.com/assets/80104/809351/492c8f9a-ee83-11e2-9587-1972ffb69904.png)