function initModel(modelName, schema) {
    if (modelCache[modelName]) throw new Error("The schema for this model has already been set");
    if (schema && typeof schema !== "object") throw new Error("Invalid Schema");

    var DEFAULT_SCHEMA = new Schema({}, {strict: false});

    setModel(modelName, schema ? new Schema(schema, {strict: true}) : DEFAULT_SCHEMA);
  }
Initializes a mongoose model with name: modelName.
If a schema is provided, it will be used to construct the model
else, the model will be initialized with a default, unrestricted
schema.

@param modelName The intended name of the model
@param schema The schema associated with this model