Calling registerFunctions and registerPartial functions properly

Calling registerFunctions and registerPartial properly by passing in the engine...

```javascript
registerFunctions(assemble.engine);
registerPartial(assemble.engine, 'partialName', content);
```

Now this can be setup properly in the assemble options of a task or target like the following:

```javascript
assemble: {
  target1: {
    registerFunctions: function(engine) {
      var helperFunctions = {};
      helperFunctions['foo'] = function() { return '<h3>bar</h3>'; };
      engine.registerFunctions(helperFunctions);
    },
    registerPartial: function(engine, name, content) {
      var tmpl = engine.compile(content);
      engine.registerPartial(name, tmpl);
    }
  }
}
```