Travis tests

- Tests from PHP 5.3 to 5.6 and HHVM
- Signers use Openssl constant (for PHP 5.4+ and HHVM)
- HHVM badge added
- Tests using PHPUnit 4.* and the last version of composer
- Code coverage information at the end of tests
- Fixed dependencies (lib-openssl and PHP version in require section)
- License added according to composer.json