Add EC2 associate_address_object method

Fixes #1874, #1893.
Make sure associate address failure returns false

Modifies EC2 connection to use common code for both associate_address and
associate_address_object. False is now properly returned for associate_address
again, keeping backward compatibility.
Merge pull request #1967 from danielgtaylor/ec2-address

Add EC2 associate_address_object method. Fixes #1874, #1893, #1967.