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!
Don't wrap the closing parentheses onto the next line, or the ContinueWith -- the whole thing can be `….ToDictionary()).ContinueWith (t => {` on the same line.