Show ascii art logo in CLI output

The aim of this PR is to make feel of PHPUnit CLI output more "sexy" and modern. The logo could also be kind of visual guide when e.g. analyzing long output from CI server and one is visually searching for start of PHPUnit output. Also some other CLI tools or competing products uses ASCII art logo. For example:

[Composer](https://github.com/composer/composer):
```
$ composer 
   ______
  / ____/___  ____ ___  ____  ____  ________  _____
 / /   / __ \/ __ `__ \/ __ \/ __ \/ ___/ _ \/ ___/
/ /___/ /_/ / / / / / / /_/ / /_/ (__  )  __/ /
\____/\____/_/ /_/ /_/ .___/\____/____/\___/_/
                    /_/
Composer version 8035dbd7147b1b97237a425e67a3513460108356 2014-06-07 15:04:06

Usage:
  [options] command [arguments]
...
```
[Nette Tester](https://github.com/nette/tester):
```
php Tester/tester.php 
 _____ ___  ___ _____ ___  ___
|_   _/ __)( __/_   _/ __)| _ )
  |_| \___ /___) |_| \___ |_|_\  v1.2.0

Usage:
    tester.php [options] [<test file> | <directory>]...
...
```

Note the Github line-spacing makes the logo preview a bit distorted - but in shell it looks nicely.