Add singleton resource support

Add support for singletons to Acfs
    
Usage:
```ruby
Single.find
# sends GET request to http://service:port/single

my_single.save
# if my_single is a new object:
#   sends POST request to http://service:port/single
# else if my_single has been requested before:
# sends PUT request to http://service:port/single

my_single.delete
# sends DELETE request to http://service:port/single
```
    
SingletonResources do not support the Resource method :all, since always only a single instance of the resource is being returned