You should be requiring Math/BigInteger.php. (It looks like Math_BigInteger is mis-named and packaged currently).
You shouldn't need to version # in your code file (you can always use the PEAR_Registry to pull currently installed version) but if you wish to have it there please use a replacement in your package.xml.
I would prefer that you use ' for all strings.
You should have a more simple example which doesn't require packages installed outside of PEAR. It is ok to keep this example, but it should be an additional example, not the main one. It should also explicitly state what else it requires and where to get it.