Added support for templated links with no arguments.

HAL API spec don't describe what's required vs. what's not in a URI parameter. These are just URI templates per https://tools.ietf.org/html/rfc6570. 

It seems incorrect to raise a `MissingURITemplateVariablesException` for a templated link because you can have something like this: `/orders{?page,size}`. Now of course we could say any `{parameter}` is required and any `{?parameter}` is optional, but that wouldn't match the URI template RFC either.

This change relaxes the requirement to specify parameters to expand a templated link, with an additional warning in the README.