Adding browser_download_url property to ReleaseAsset Class

Hello, I'm taking a swing at Issue https://github.com/octokit/octokit.net/issues/599. 
I'm also looking into a Test for this but notice that 

```csharp
<ReleaseAsset> GetAssets
``` 
That is used in 

```csharp
ReleaseClient
``` 
returns null parameters. 

Any input is appreciated. 

Fixes #599