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
Merge pull request #3 from cddr/setup-docs

Setup docs
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
Merge pull request #4 from cddr/hal-schema

Add schema vars for HAL entities
Fix indentation in gitlog-to-changelog script