Add HTML whitespace remover

Use case:

HTML document 1: http://resurtm.com/yii-stuff/spaceless-use-case-1.html
HTML document 2: http://resurtm.com/yii-stuff/spaceless-use-case-2.html

Diff between them:

```
$ diff spaceless-use-case-1.html spaceless-use-case-2.html
43c43,44
<               </div><div class="right">
---
>               </div>
>               <div class="right">
```

Patch provided in this PR would allow us to solve this problem as follows:

```html
<?php \yii\helpers\Html::spaceless(); ?>
<body>
	<div class="wrapper">
		<div class="left">
			<ul>
				<li><a href="#">Posts</a></li>
				<li><a href="#">Users</a></li>
				<li><a href="#">Contact Us</a></li>
				<li><a href="#">Legal Notes</a></li>
			</ul>
		</div>

		<div class="right">
			<p>Even when the Xbox One is in sleep mode, its built-in microphone can always listen in. It’s a feature developers say will provide quick voice-command access to games and apps — but that could spook commanders who might worry the always-connected device could also capture more than just idle chit-chat among troops.</p>

			<p>“Microsoft has single handedly alienated the entire military. And not just the U.S. military — the militaries of the entire world,” says naval aviator Jay Johnson.</p>

			<p>The Internet connection requirement is “the single greatest sin Microsoft has committed against all service members,” he writes in a post on the game developers’ site Gamasutra.</p>

			<p>Sample text taken from here:<br/>http://www.navytimes.com/article/20130614/OFFDUTY02/306140030</p>
		</div>
	</div>
</body>
<?php \yii\helpers\Html::endSpaceless(); ?>
```