Feature/elastic ip enhanced

Further refinements to Elastic IPs; These two symbols, :auto_elastic_ip and :elastic_ip are 100% mutually exclusive. 

NOTE: Trying to set both on any single facet definition will cause Ironfan to throw a fatal error. This includes having set one in the global ` cloud(:ec2) do ` block and then trying to set the other in a facet block.
 
Sample Usage:
```ruby
facet :vpn do
  cloud(:ec2) do
    # for using the first available Elastic IP for our domain/scope
    auto_elastic_ip 'first'
  end
end
```
--- or ---  
```ruby
facet :vpc do
  cloud(:ec2) do
    vpc                 'vpc-xxxxxxxx'
    subnet              'subnet-xxxxxxx'
    # for using known, preallocated Elastic IPs; best for
    # attaching to VPCs.
    elastic_ip          'xxx.xxx.xxx.xxx'
  end
end
```