Modify Jetpack_Client_Server to support unit testing, add some tests.

Here's a proposal for modifying the Jetpack_Client_Server class so that it can be unit tested. Does this seem reasonable?
Ah, you're totally right. The issue is that Jetpack::init() is static though? I switched up the code here to look like this:

```
	private static $jetpack;

	function __construct() {
		self::$jetpack = Jetpack::init();
	}
```
and
```
	public static function set_jetpack( $new_jetpack ) {
		self::$jetpack = $new_jetpack;
	}

	public static function get_jetpack() {
		return self::$jetpack;
	}
```
and added test_jetpack_client_server_get_jetpack_testable for sanity. That allows us to switch out the static reference even if we can't stub the function. Does that seem better? I'm not sure how else to address it. :(