Allow user-specified default options for app.js

I came across an issue where I needed the automatic initialization at `app.js` to pick up a particular option for the `foundationTabs` plugin, but I found no way of doing this but getting rid of this automatic behavior and doing everything by hand -- which kind of took away some of the magic of using Foundation :disappointed:

So, I'm sending this PR with a sort of new feature: allowing for user-specified default options that will be picked up by `app.js`.

## Here's how it works:

1. Have a script executed **before** the on-ready callback at `app.js` which sets a hash at `$.foundationDefaults` with the desired defaults. More on how to do this later.
2. Once `app.js` executes its on-ready callback, it will try to pick up the settings and initialize each plugin with the defaults specified for it.

### Specifying per-plugin defaults

All plugin defaults are namespaced under their plugin name (without the `foundation` prefix, as it would be dreadfully repeating). Defaults for any plugin can be specified without having to specify anything the rest of them:

```javascript
// Before jQuery.ready callbacks are executed
$.foundationDefaults = {
  tooltip: { tooltipClass: '.not-your-mamas-tooltip' }
}
```

### What about the *default defaults*?

`app.js` also has a `defaults` variable which is currently set to the only *default default* option I found: `callback` for `tabs` plugin:

```javascript
defaults = {
  tabs: {callback : $.foundation.customForms.appendCustomMarkup}
}
```

### An example (Rails app)

With a simple Rails project, you might add the user default settings at (for the sake of this example's simplicity) `application.js`:

```javascript
//
// application.js: Application-wide scripts.
//
//= require jquery
//= require jquery_ujs
//= require foundation
//= require_tree .

jQuery.foundationDefaults = {
  tooltip: { tooltipClass: '.not-your-mamas-tooltip' }
};
```

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