Updating to always register default helper-lib and how registerFunctions method works

Updating to always register the default helper-lib helpers.

Updating registerFunctions to take in an object and iterate over the keys to load the helpers. Usage:

```javascript
var engine = require('assemble-handlebars');
var helpers = {
  foo: function() { return '<h3>bar</h3>'; },
  bar: function() { return '<span>baz</span>'; }
};

engine.registerFunctions(helpers);
```