externalise the dependencies such as BigInteger.php and stand.php

Now that the code is compatible with composer and published on packagist we should be able to remove the BigInteger math package which originally came from Pear and reference one on packagist. Also the srand.php was defensive logic around buggy random number generators in old php distros. There is likely to be something better supported on packagist that I should add to the composer.json.