Adds namespace support

[charlesportwoodii]

Adds support for namespaces to differentiate this further from the PHP NEXT implementation.

\Argon2\Hash(string $string [, const $algorithm = HASH_ARGON2ID] [, array $options ] [, bool $raw = false ]);
\Argon2\Verify(string $string, string $hash);
\Argon2\Info(string $hash);