Refactor ExceptionNotification to accept notifiers

The ExceptionNotifier middleware nows accepts multiple notifiers.

Notifiers are objects which respond to method `#call(exception, options)`
in order to execute some action: send an email, publish to a chat room,
call a webhook or whatever your crazy little mind wants!

You can configure your notifiers passing a hash of options to the
ExceptionNotifier middleware in the given form:

    use ExceptionNotifier,
      :email => { ... email options ... },
      :campfire => { ... campfire options ... },
      ...

Though, to this thing work, notifiers MUST BE implemented under the
ExceptionNotifier namespace and MUST HAVE a name ending 'Notifier'. e.g:
'ExceptionNotifier::EmailNotifier'.

Moreover, any notifier can be added/removed later in some place of
the application, like a Rails initializer, using the method
'ExceptionNotifier#register_exception_notifier(name, notifier)' since
the `notifier` responds to `#call(exception, options)` method. Example:

    ExceptionNotifier.register_exception_notifier :test,
      lambda { |exception, options| ... do something ...  }

By now, the only notifier implemented is EmailNotifier.