Exception catchers

The initial purpose of this branch was separate notification logic from the rack middleware, both implemented in ExceptionNotifier class. But things got very simpler and better. :bowtie:

### Changelog

- [x] split out rack middleware from exception notifier core
  * Creates [ExceptionNotification::Rack](https://github.com/jweslley/exception_notification/blob/exception_catchers/lib/exception_notification/rack.rb) class which implements the rack middleware. Thus, it became easier to add others 'exception catchers' like [resque](https://github.com/jweslley/exception_notification/blob/exception_catchers/lib/exception_notification/resque.rb) and [sidekiq](https://github.com/jweslley/exception_notification/blob/exception_catchers/lib/exception_notification/sidekiq.rb). Because this change, now configuration must look like:

```ruby
config.middleware.use ExceptionNotification::Rack,
  :email => {
    :email_prefix => "[ERROR] ",
    :sender_address => %{"notifier" <notifier@example.com>},
    :exception_recipients => %w{exceptions@example.com},
  }
  ...
```

- [x] add resque support
  * errors occurred in resque jobs can be automatically sent
- [x] add sidekiq support
  * errors occurred in sidekiq jobs can be automatically sent
- [x] add rails engine
  * automatically adds the rack middleware in rails applications
- [x] add Rails generator to create an [initializer file](https://github.com/jweslley/exception_notification/blob/exception_catchers/lib/generators/exception_notification/templates/exception_notification.rb)
- [x] remove residual logic from the rack middleware
  * avoid unexpected behavior when triggering exceptions manually (like this #159, which occurs because the crawler logic is present in the rack middleware)
- [x] add logger to log when notifications cannot be shiped
- [x] Update docs (README/Changelog)

/cc @smartinez87