Add wrapper script for knife-cloudformation deployments

This change adds a new ruby script called `signifyd-cf-deploy` in the bin directory. This script accepts all the same arguments that are being passed to knife-cloudformation in the current build, so it should be a drop-in replacement.

There are probably some corner cases we've yet to discover, but I think this should be serviceable for the short term. Caveats include the following:
1. An ELB stack must be manually pre-provisioned and the ELB stack name must be passed as the value for the `--apply-stack` flag
2. Names for application stacks deployed using this wrapper must follow the pattern `app_name-environment-build_number` (e.g. `platform-test-639` or `platform-prod-735`)
3. --All instances registered in the ELB must be status "InService" before we consider the new stack deployment successful.--
4. The script only destroys a previous stack when it finds one (and only one) stack matching the same `app_name-environment-build_number` pattern.
5. The script does not enforce directionality on build numbers. i.e. given an existing stack named `platform-test-639`, the script will happily deploy a new stack named `platform-test-637` and then destroy `platform-test-639`.