Packer file that creates VirtualBox, VMWare and Amazon EC2 VMs from the ...

...Cypress build script

Updated gitignore to ignore Packer cache/output

Updated contrib README to include cypress.json

Added Packer-built tarballs to gitignore

Added Preseed file for Packer VM bootstrapping

Added compressor post-processors for the two VM builders

Added environment variable check to install_cypress.sh that checks for unattended-install