\Phalcon\Debug\Dump + tests; removed Firephp logger and formatter + some minor additions

1. added \Phalcon\Test\ModelTestCase::truncateTable() method, which should be used when deleteTable is making trouble because of foreign keys

2. removed \Phalcon\Logger\Adapter\Firephp and \Phalcon\Logger\Formatter\Firephp since they are part of Phalcons core now

3. added \Phalcon\Debug\Dump component for easier variable dumping within Phalcon application

4. added unit tests for \Phalcon\Debug\Dump (tests/Phalcon/Debug/DumpTest.php)

5. added phpunit/phpunit to composer.json require-dev

6. added vendor/ folder to .gitignore