Explicit `JobStorage` instance for Dashboard

This PR fixes #168. Added overload to the `MapHangfireDashboard` extension method of the `IAppBuilder` interface. Usage is simple (please see [this file](https://github.com/HangfireIO/Hangfire/blob/master/samples/MvcSample/Startup.cs) for the details):

```csharp
app.MapHangfireDashboard(
    "/hangfire-redis", 
    new IAuthorizationFilter[0],
    new RedisStorage("localhost:6379", 3));
```