split filter does not work

[urkle]

When trying to use the split filter it throws an exception that split is not implemented for StringValue.

1. it seems that the catch of split was never added to the case statement in Interpreter.php's applyFilter method. (surely there is a more generic way of implementing this?)
2. once I added an a case for 'split' into, there I found that the implementation is broken as it defaults the "splits" to IntegerValue(-1) which means "all but the last". It should be IntegerValue(PHP_INT_MAX) so that ALL splits are returned.
3. tests really should be added for ALL the filter types to ensure that 100% of the functionality works.