Minor clarifications for using JS module format

Changes some documentation and text strings surrounding the JS module
input format:

- differentiates use of dynamic module files in the introduction against
  using the static data formats
- clarify that URLs can only be used for JSON and YAML input, not for JS
  modules
- updates exception in _getDataFromFile() to mention JS modules as an
  acceptable input format
- adjust the unit test message for the JS module format

README.md

@@ -3,7 +3,7 @@
 [![Build Status](https://travis-ci.org/5thWall/mustache-render.png?branch=master)](https://travis-ci.org/5thWall/mustache-render)
 [![NPM version](https://badge.fury.io/js/grunt-mustache-render.png)](http://badge.fury.io/js/grunt-mustache-render)
 
-This is a grunt plugin to render [mustache](http://mustache.github.io/) templates. It takes data in `JSON`, `YAML`, `JS` or `POJO` (Plain Ol' JavaScript Object) format. It allows you to specify a folder for partials, instead of needing to list them individually.
+This is a grunt plugin to render [mustache](http://mustache.github.io/) templates. It takes data in static `JSON`, static `YAML`, `JS` module, or a `POJO` (Plain Ol' JavaScript Object) format. It allows you to specify a folder for partials, instead of needing to list them individually.
 
 ## Getting Started
 This plugin requires Grunt `~0.4.1`
@@ -37,7 +37,7 @@ grunt.initConfig({
       },
       files : [
         {
-          data: // Path or URL to JS, JSON or YAML file, or POJO
+          data: // Path to JS module, path or URL to JSON or YAML, or POJO
           template: // Path or URL to template file
           dest: // Path to output destination here
         }

tasks/mustache_render.js

@@ -160,7 +160,8 @@ module.exports = function gruntTask(grunt) {
       return require(path.resolve('.', dataPath));
     }
 
-    throw new Error("Data file must be JSON or YAML. Given: " + dataPath);
+    throw new Error("Data file must be JSON file, YAML file, or JS module. " +
+                    "Given: " + dataPath);
   };
 
   // Internal: Ensure template is in proper format and retrieve its body.

test/mustache_render_test.js

@@ -106,7 +106,7 @@ exports.mustache_render = {
     var actual = grunt.file.read('tmp/hello_js.html');
     var expected = grunt.file.read('test/expected/hello_world.html');
 
-    test.equal(actual, expected, 'should render when given arbitrary data.');
+    test.equal(actual, expected, 'should render when given a JavaScript module.');
 
     test.done();
   },