Provide a high level API to simplify usage

[lcobucci]

@Ocramius gave a nice idea to help users: based on the Configuration have a really simple way to get a valid token or null from a raw token (string), something like:

function (string $tokenData) use ($configuration): ?Token {
    $parser = $configuration->getParser();
    $validator = $configuration->getValidator();
    $constraints = $configuration->getConstraints();

    try {
        $token = $parser->parse($tokenData);
    } catch (\Lcobucci\JWT\Exception $e) {
        return null;
    }

    if (!$validator->validate($token, ...$constraints) {
        return null;
    }

    return $token;
};

In order to achieve this we would need to implement #139 but also add the validation constraints in the configuration object.

[lcobucci]

Depends on #129 (sure)