Recurring jobs with HangFire

Closes #62. Creating and running recurring job inside ASP.NET application is now as simple, as one line of code:

```csharp
RecurringJob.AddOrUpdate(() => Console.WriteLine("This is awesome!"), Cron.Daily);
```

It uses the [NCrontab](https://code.google.com/p/ncrontab/) library, so you can specify the schedule via [CRON expressions](http://en.wikipedia.org/wiki/Cron#CRON_expression):

```csharp
RecurringJob.AddOrUpdate(() => Console.WriteLine("This is incredible!"), "0 12 * */2");
```

Recurring jobs are being created as regular background jobs, so you receive all other features of HangFire for free.

TODO:

* [ ] Finish the scheduler class, add tests.
* [ ] Add support for recurring job deletion.
* [ ] Add different overloads to the `RecurringJob` class.
* [ ] Display recurring jobs in HangFire Monitor.