Add phpunit to composer.json

Use phpunit from Packagist.
Do `composer update` to install phpunit in this project.

* phpunit/phpunit

* version 3.7.~

* vendor/bin/phpunit
Add MarkdownTest.php

This unit test is check how working `michelf/php-markdown`.

* Add testHeader()

* Add testStrong()
Merge pull request #21 from toiee/test-markdown

Add MarkdownTest.php and phpunit to composer.json