Module packages

[ericzbeard]

This PR adds package aliases to the Rain section, allowing module URIs to be aliased. It also adds the capability to unzip a directory of modules and verify a hash.

Rain:
  Packages:
    aws:
      Location: https://github.com/aws-cloudformation/rain/modules
    xyz:
      Location: ./my-modules
    abc:
      Location: https://github.com/aws-cloudformation/rain/releases/tag/m0.1.0/modules.zip
      Hash: https://github.com/aws-cloudformation/rain/releases/tag/m0.1.0/modules.sha256

Resources:
  Foo:
    Type: !Rain::Module aws/foo.yaml

  Bar:
    Type: !Rain::Module xyz/bar.yaml

  Baz:
    Type: $abc/baz.yaml
    # Shorthand for !Rain::Module abc/baz.yaml

Modules from the test directory were moved to the top level and will be released separately with their own version number. This allows the package to be referenced by tags, such as m0.1.0 as shown in the example above. The major version number will be incremented if any breaking changes are introduced to the modules. The available modules in the release package are listed below.

This PR also adds a few other features, like putting Constants in modules, and conditionally emitting resources in modules with the IfParam Rain Metadata attribute.