Add VPC support for EIP module

This is to fix Issue #4436

This PR adds support for:

* creating EIPs inside a VPC
* associating an EIP with an EC2 instance (inside and outside VPC)
* disassociating an EIP from an EC2 instance (inside and outside VPC)
* releasing an EIP (both inside and outside a VPC)
* catching all Boto exceptions and failing gracefully

Please let me know if you have any comments or feedback on this.