Implemented CalendarFolder#create_item using a CalendarItem template.

I created an easy to use method for creating calendar items.

Usage example:
```ruby
ews_client = Viewpoint::EWSClient.new endpoint, username, password
folder = ews_client.get_folder :calendar
appointment = folder.create_item(subject: 'My Appointment', start: Time.now)
puts appointment.item_id # => {:id => 'ABC...', :change_key => 'def...'}
```