Changed parameters to accept IUnityContainer

It's nice to ask for a Interface instead of the concrete
implementation in method parameters. Since the package
Unity.AspNet.WebApi uses the interface IUnityContainer to configure a Unity container
in a WebApi, we should also use the interface IUnityContainer in
HangFire. And its compatible with clients already using this package.

So instead of:
```c#
config.UseUnityActivator(UnityConfig.GetConfiguredContainer() as Microsoft.Practices.Unity.UnityContainer);
```
now i can use:

```c#
  config.UseUnityActivator(UnityConfig.GetConfiguredContainer());
```