Use vendored resources, improve test coverage

This PR does the following:

- Allows Omnibus to fetch local files (inside the gem) instead of requiring they exist inside the project
- Prefers local project files
- Adds flags to the generators to allow generating dmg, pkg, and msi assets
- Disables generating dmg, pkg, and msi assets by default
- Adds unity-functional tests for many of the packagers
- Removes `Project#files_path` in favor of `Project#resources_path`
- Standardizes on the term "resource" to refer to a template, asset, or image in a package

/cc @opscode/release-engineers 

@scotthain this does **not** fix your AIX issues, but it gets us much closer. Once we get this merged in, I can start working on refactoring the BFF packager to use this new format and add test coverage.
@yzl I have a `NOTE` in the packages to "make this configurable". I definitely think we should. I've created https://github.com/opscode/omnibus/issues/299 to track that work, but I'd like to keep it separate from this (since this is already huge).