Add a new sniff "Generic.PHP.DisallowMultiplePHPTags"

Hey everybody,

### Introduction

This sniffs was created during the development for [TYPO3](http://typo3.org/) / [FLOW3](http://flow3.typo3.org/) [CodingStandard](http://forge.typo3.org/projects/team-php_codesniffer).
Due to our previous contact via email, we have renamed them to implement this into the Generic Standard. This sniff is backed by a unit test and are well tested by our [Jenkins installation](https://ci.typo3.org/) and own every day usage.

#### New sniff

* Generic.PHP.DisallowMultiplePHPTags

### Explaination of sniff

#### Generic.PHP.DisallowMultiplePHPTags
**Description**: Only one pair of PHP-Tags in one file
**UnitTest**: Yep, included.
**Example**: 

Wrong:
```php
<?php
doSomething();
?>
<span>some html content</span>
<?php 
doSomethingMore(); 
?>
```
Correct:
```php
<?php
doSomething();
echo '<span>some html content</span>';
doSomethingMore(); 
?>
```