Document references.

This adds documentation for references, e.g. an EC2 instance has a `vpc`
and a `subnet` reference. This also documents reverse references discovered
through the `subResources` association, such as an S3 object having a `bucket`
reference. Previously these were not documented.

Note: this does **not** take in to account #17, but will be modified to do so
in the future. Right now only one reference is affected: `VpcPeeringConnection.vpc`
has been renamed to `VpcPeeringConnection.vpc_reference` but this is not
reflected in the docs yet. It's on my short-list to fix but requires larger changes
to the codebase.

cc @jamesls, @kyleknap