Added support for attaching EBS snapshot when creating a new instance

[mfojtik]

Hi,

This patch will allow you to attach EBS snapshot when you're creating an instance.
BlockMappings schema was changed in EC2 query API so old way doesn't work anymore.

Syntax for attaching snapshots:

client.launch_instances('ami-8c1fece5', :instance_type => 'm1.small', 
  :block_device_mappings => [ {
  :virtual_name => 'test1',
  :ebs_snapshot_id => 'snap-3d860552',
  :device_name => '/dev/sdc'
} ] )

-- Michal