[aws|elb] add support for ELB connection draining

Add support for ELB connection draining: http://docs.aws.amazon.com/ElasticLoadBalancing/latest/DeveloperGuide/config-conn-drain.html

This allows you to view whether Connection Draining is enabled or disabled, and also to view the Timeout setting. You can also toggle the feature on/off, and change its Timeout setting.

The relevant tests pass locally for both `FOG_MOCK=true`. I tried running them with mocking disabled, but ran into the following issue:

```
  AWS::ELB | policy_tests (aws, elb) +++++++++++++++++++  
  AWS::ELB | models (aws, elb) +++++++++++++++++          
          /Users/bgentry/Code/fog/tests/aws/models/elb/model_tests.rb
            with vpc
            create
            success
            AWS::ELB | models (aws, elb)
          undefined method `ec2_compatibility_mode' for #<Fog::Compute::AWS::Real:0x007ff08eba0618> (NoMethodError)
            /Users/bgentry/Code/fog/tests/aws/models/elb/model_tests.rb:61:in `block (4 levels) in <top (required)>'
            /usr/local/opt/rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/shindo-0.3.8/lib/shindo.rb:79:in `instance_eval'
            /usr/local/opt/rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/shindo-0.3.8/lib/shindo.rb:79:in `tests'
            /Users/bgentry/Code/fog/tests/aws/models/elb/model_tests.rb:60:in `block (3 levels) in <top (required)>'
```

When I added a stub for that method to `Fog::Compute::AWS::Real`, I still had some very strange results from unrelated tests. Maybe it's because I have an old (non-VPC default) AWS account? Maybe somebody else can run those tests with `FOG_MOCK=false` ?

/cc @mikehale @pedro