Feature/timezone support

# Summary

Hi there,

With this pull request, I'm proposing to add moment-timezone support.

This would allow the `amCalendar` and `amDateFormat` filters to display dates aligned to a different timezone instead of the local machine's.

# How?

To configure this, a new `angularMomentConfig` constant was created as follow:

````javascript
angularMoment.constant('angularMomentConfig', {
	timezone: '' // e.g. 'Europe/London'
})
````

I took also the initiative to regroup the code inside a closure to:

- define the `applyTimezone` private function
- optimise the `'use strict'` definition

# Usage example

**HTML**

```html
<script src="components/moment/moment.js"></script>
<script src="components/moment-timezone/moment-timezone.js"></script>
<!-- Visit momentjs.com/timezone/data/ to obtain your own timezone data -->
<script src="your_own/moment-timezone-data.js"></script>
<script src="components/angular-moment/angular-moment.js"></script>

<!-- 
	For a message.time = new Date('2013-10-07T00:00:00.000Z') 
	and angularMomentConfig.timezone = 'Australia/Sydney'
-->

<span>{{message.time | amDateFormat:'dddd, MMMM Do YYYY, h:mm:ss a'}}</span>

<!-- This snippet will format the given time as "Monday, October 7th 2013, 11:00:00 am". -->
```

**Javascript**
```js
var myapp = angular.module('myapp', ['angularMoment']);

myapp.controller('app', [
	'angularMomentConfig',
	function (angularMomentConfig) {
		angularMomentConfig.timezone = 'Australia/Sydney';
	}
]);
```