PHP 5.3+: Replace custom autoloader with composer autoloader.
Add PSR4 namespace to composer.json.
Use __construct.
Add namespace statement. Rename class.
Remove any include statement.
Use namespace in example code.
Reference to instances as BigInteger instead of Math_BigInteger.
Replace new Math_BigInteger with new static.
Adjust test cases using Math_BigInteger.
Some more documentation polishing in Math/BigInteger.php.
Update users of Math_BigInteger.