Add the ability to resize the root volume

adds a -x parameter.  This causes some problems if you wish to undo the
damage, so to speak at a later date because the code will preserve the
old ebs volumes.
Merge pull request #77 from vjanelle/ec2autoimage/resize-root-fs

Add the ability to resize the root volume.
Works as it stands ... note that it will only update volume size when the AMI id also changes.