[AWS|ELB] policy model update

Add the ability to create and view non "stickiness" policies through the policy models.

```ruby
elb.policies.create(:id => "my-public-key-policy", :type_name => 'PublicKeyPolicyType', :policy_attributes => {'PublicKey' => AWS::IAM::SERVER_CERT_PUBLIC_KEY})
elb.policies.get("my-public-key-policy")
=> <Fog::AWS::ELB::Policy
     id="my-public-key-policy",
     cookie=nil,
     expiration=nil,
     type_name=nil,
     policy_attributes=nil
    >
```

Currently the `DescribeLoadBalancersResult` does not include any details about other policies, that would have to be fetched separately with a call to `DescribeLoadBalancerPolicies` for the elb in question. What are thoughts on the current state of this? Should the extra call be included for the model in the case where `OtherPolicies` are present on the elb?