Owin support for HangFire Monitor

Closes #82. Provides OWIN based pipeline for HangFire Dashboard (known as Monitor). This is the last feature I want to present before the `1.0` version to lower the dependency burden for extensions.

### Features

* No dependency hell for extension packages that want to include both core logic and dashboard pages. For example, recurring jobs can now be implemented as a single package.
* Tight integration with other OWIN based frameworks, such as [FubuMvc](http://mvc.fubu-project.org/), [Nancy](http://nancyfx.org/) and [Simple.Web](https://github.com/markrendle/Simple.Web).
* All parts of HangFire can now be hosted anywhere.
* Simplified unit/integration testing for the dashboard.
* No `web.config` transformations – you can configure all features through the code.
* Claims/Roles based, as well as custom authorization support through the `IAuthorizationFilter` interface.

### Breaking changes

* HangFire now requires .NET Framework 4.5.
* `HangFire.Web` package and assembly merged with `HangFire.Core`, since Owin components does not rely on anything.
* Removed `AspNetBackgroundJobServer` class, `Microsoft.Owin.Host.SystemWeb` package supports shutdown request notifications via `host.OnAppDisposing` environment value.
* Default dashboard url changed to `/hangfire` (removed `.axd` suffix).
* Changed dashboard authorization configuration.

### TODOs

* [x] Local request only / custom authorization for dashboard.
* [x] Single entry for the whole HangFire – `app.UseHangFire();`.
* [x] Fluent configuration for all components based on `IAppBuilder` interface for web applications.
* [x] NuGet package updates. Change `HangFire` to include `Microsoft.Owin.Host.SystemWeb` dependency.