Allow specifying an alternate template namespace.

Allows precompiling HTMLBars templates.

```javascript
emberTemplates: {
  options: {
    templateCompilerPath: 'bower_components/ember/ember-template-compiler.js',
    handlebarsPath: 'bower_components/handlebars/handlebars.js',
    templateNamespace: 'HTMLBars'
  },
  files: {
    'tmp/templates.js': 'app/templates/**/*.hbs'
  }
}
```
Merge pull request #77 from rwjblue/allow-htmlbars-precompilation

Allow specifying an alternate template namespace.