Add support for generating API documentation

To generate project docs using codox, run

```
    $ lein doc
```

On pushes to the github repo, travis will attempt to generate the docs
and the tests will fail if [test/tooling.sh]
(https://github.com/cddr/schema.contrib/blob/master/test/tooling.sh)
returns a non-zero exit code
Add basic HAL types

Add schema vars for the basic HAL types of Resource, Link, and
Curie. See the [HAL Specification][hal] for full details

[hal]: http://tools.ietf.org/html/draft-kelly-json-hal-06
Use the markdown codox plugin to support links in doc strings
Fix indentation in gitlog-to-changelog script