Generic hashing framework for PHP

Simplest usages:

$digest = hash($algoname, $message);
$digest = hash_file($algoname, $filename);

Examples:

$digest = hash('md5', 'The quick brown fox jumped over the lazy dog.');

Feeder usage:

$context = hash_init($algoname);
hash_update($context, $message);
$digest = hash_final($context);

hash(), hash_file(), and hash_final() each support an optional boolean parameter $raw_output which behaves in the same
manner as sha1()'s optional parameter.
