It requires a allocation_id when associating the VPC instance to an EIP.

I found a bug inside boto.ec2.address.Address.associate method. 
It requires a allocation_id when associating the VPC instance to an EIP.

So, here is my fix:

1. If there is allocation_id for this address, provide it as a parameter, when calling associate_address.
2. allow passing 'allow_reassociation' to associate_address.