add source indexing using SourceLink

Source indexing the pdb files and including them in the nupkg files allows Visual Studio and other clients to download the source files on demand from GitHub. Several open source and closed source projects are using my project, [SourceLink](https://github.com/ctaggart/SourceLink) to source index their projects.

/to @Haacked & GitHub
I would like this solution to work for our private repositories on GitHub, but we need a compatible authentication mechanism for raw content that works with `srcsrv.dll` in Windows. I'm at the Microsoft MVP Summit in case anyone wishes to chat in person about this.


![image](https://cloud.githubusercontent.com/assets/80104/4905769/20cf5ed2-6453-11e4-9b0d-bb6a394c7e80.png)

```
PS C:\Projects\octokit.net> srctool -x "C:\Projects\octokit.net\packaging\Octokit.Reactive.0.5.0\lib\net45\Octokit.React
ive.dll"
https://raw.githubusercontent.com/octokit/octokit.net/293f0f22345f5bd9f75f4ec64f1ced0351e3a8f0/Octokit/Helpers/Ensure.cs

https://raw.githubusercontent.com/octokit/octokit.net/293f0f22345f5bd9f75f4ec64f1ced0351e3a8f0/Octokit.Reactive/Clients/ObservableRepositoryDeployKeysClient.cs
https://raw.githubusercontent.com/octokit/octokit.net/293f0f22345f5bd9f75f4ec64f1ced0351e3a8f0/Octokit.Reactive/Clients/ObservableOauthClient.cs
https://raw.githubusercontent.com/octokit/octokit.net/293f0f22345f5bd9f75f4ec64f1ced0351e3a8f0/Octokit.Reactive/Clients/ObservableRepositoryCommentsClient.cs
https://raw.githubusercontent.com/octokit/octokit.net/293f0f22345f5bd9f75f4ec64f1ced0351e3a8f0/Octokit.Reactive/Clients/ObservableDeploymentsClient.cs
...
```
I'm happy to let that error bubble up to the user. :thumbsup: to dropping this check.