Add Bug Search Options

I added BugSearchOptions and GetBugSearchResultsAsync. 

The API is pretty simple, here's an example:
```csharp
var client = new BugzillaClient (new Uri("https://bugzilla.your-company.com/jsonrpc.cgi"));
var bso = new BugSearchOptions(){
	Status = "RESOLVED",
	Resolution = "FIXED",
	Limit = 100
};

var bugs = client.GetBugSearchResultsAsync(bso).Result;
foreach (var bug in bugs)
	Console.WriteLine (bug.Id);

```

I handled InvalidCastExceptions when parsing the json into a bug, and I added a warning about not seeing private data when user is not logged into an account. 

Happy to take comments and modify, but this works great for me!