Description:
------------
Add a deleteMin() method to SplMaxHeap (and/or a deleteMax() to 
SplMinHeap) to limit the size of the heap without taking off the top 
element.

Reproduce code:
---------------
$heap = new SplMaxHeap();
for ($i = 0; $i < 10000; $i++) {
$heap.insert($arr[$i])
if ($heap.count() > 100) {
$heap.deleteMin();
}
}

Expected result:
----------------
The example is pretty useless, it will add 100 elements to the heap and 
then add 9900 more, immediately removing each one.

This is useful in other situations where you may only be interested in 
the top 100 sorted elements of 10000 unsorted elements and don't want 
the heap to grow excessively large.

Actual result:
--------------
There is no deleteMin() method in SplMaxHeap