Travis tests

- Tests from PHP 5.3 to 5.6 and HHVM
- Signers use Openssl constant (for PHP 5.4+ and HHVM)
- 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
- Documentation updated:
    * HHVM badge added
    * PHP 5.2 is not supported. This library requires at least PHP 5.3
- PHP-CS-Fixer used to update all files in this library (PSR-2 level)
- New signers added according to the JWA Draft 27 (see
http://tools.ietf.org/html/draft-ietf-jose-json-web-algorithms-27):
    * HS256
    * HS384
    * HS512
    * none
- Tests added for new signers
- hash function removed in RSA.php since this function is never used and
not defined by the interface
- composer.json updated:
    * Test namespace removed. This namespace is now added only when tests
are executed