Creating Templating System

This is the first go at the templating system in #146. 

You can copy *any* or *all* of the files in the `ninja-forms/templates/` directory into your theme. So if you wanted to override the `ninja-forms/templates/fields/help.php` file you would have to put it in `your-theme/ninja-forms/fields/help.php`.

Since this is a proof of concept it only includes:

* form/open-form-tag.php
* form/close-form-tag.php
* fields/help.php

If we decide to go ahead with this we should add all of the fields.

Here's an example of these fields being overridden to add an extra paragraph, add a wrapper div, and add a div around the help field.

![ninja-form-templates](https://f.cloud.github.com/assets/1065372/1924608/f702f754-7e24-11e3-9767-7decae0f40de.png)