Make template path configurable.
Add built-in template helper 'render', invoked as `{{render foo .}}`. The helper will first look for foo.html in the subdirectory of the templates directory corresponding to the controller responding to the current request, and if absent, will then look for foo.html in the root templates directory.

As a result of instrumenting the broker for testing, the path where the broker looks for templates is now configurable by setting the value of `templates.TemplatePath`.

This resolves #18.