Memory management and Page Counting

1. Added a PagerState.Release() statement into the constructor of the PureMemoryPager class.  
2. Added tests to ensure that the creation and disposal of pagers leaves the instance count unaffected.
3. Modified the Tree.AllPages() method to account for overflow pages.
4. Added lines into the CanAddVeryLargeValue() test to compare the results of AllPages.Count() with the PageCount state variable.
5. Corrected typo in spelling of Overflow.