#3 Add basic filters

[Paul Chandler]

Add some basic xrange-compatible filtering iterators to enhance usage. Some potential ideas:
EvenFilterIterator
OddFilterIterator
PerfectSquareFilterIterator
NumericFilterIterator
RangeFilterIterator
LambdaFilterIterator

[Paul Chandler]

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OddFilterIterator and EvenFilterIterator are checked in as of revision 12. You can now do this:
// grab the second & third even numbers from 1 .. 10 ... 4 & 6
$i = new LimitIterator(new EvenFilterIterator(xrange(1, 10)), 1, 2);

[Paul Chandler]

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Added NumericFilterIterator, which is an iterator that filters as an is_numeric() call.