signer works again with string public keys

returning instance to allow method chaining

See https://github.com/namshi/jose/issues/15#issuecomment-67822189

Adds ES256, ES384, ES512 signer support

Fix bug with PHP<5.4.8

Because constants were introduced in PHP 5.4.8, RSA signer will not work
when using PHP < 5.4.8.
This commit fixes the issue #10

fixed some typos in the README

Add url safe base64 encoding

BC safe with fallback to the old way of encoding

Add some phpdoc/code style corrections

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
- Documentation updated:
* 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

The PHP 5.5 build is now required to make the travis build pass

adding Issued At 'iat' to payload array