add model API layer foundation

* each model class has a corresponding *Api.js mixin module, whose
  exported functions are copied to the model class prototype on server
  startup
* API functions appear on separate jsDoc pages through @mixin/@mixes
* the "global" model layer API is in a separate module (globalApi.js)
* the API functions are generally intended to be just a thin layer
  above existing model functionality, hence they should not normally
  require additional tests
* the first batch of API functions added here mostly covers
  functionality already implemented in the model classes, and some stubs
  for functions that are used during Player creation (which was used
  as the test use-case while implementing the API layer)
Merge pull request #18 from yelworc/model-api-layer

add model API layer foundation