Add resource path resolver for macOS

[yuhr]

Don't merge yet! This PR is an early-bird for #12, being not yet publicly usable because servo/core-foundation-rs#416 is not yet officially released. Just tracking purpose for now, although you can try it by adding following dependency to your project's Cargo.toml.

cargo-bundle = { git = "https://github.com/yuhr/cargo-bundle", branch = "cross-platform-resource-loading" }

I've added a companion library which publishes two functions resources_root and resource_path, but the APIs may change due to compatibility with other platform-specific stuff.

Example:

let path = cargo_bundle::resource_path(path_relative_from_project_root);

[yuhr]

macOS has an intrinsic system to bundle some independent (I mean, not embedded into executable binary) resources and metadata files, while others do not AFAIK. Current windows support is effectively broken as noted in #77, #78, and #79, so I can't see where bundled resources stored actually but maybe next to the executable would be appropriate...

[fwcd]

Any updates on this? If I read this correctly, an updated version of core-foundation-rs should be released.