Convert singleton calculation engine to multiton
Ensure that writers access the workbook rather than the global calculation engine
Use direct reference to workbook in Calc Engine rather than cell's parent's parent
Eliminate some code duplication
Selective calculation cache clearance and adjustment for unsetting or renaming worksheets
Merge branch 'develop' of https://github.com/PHPOffice/PHPExcel into calcEngine

Conflicts:
	Classes/PHPExcel.php
	Classes/PHPExcel/Worksheet.php
Fix PHPExcel object destructor
Refactoring of calculation engine using the multiton pattern to eliminate caching issues when working with multiple workbooks
Refactoring of calculation engine for improved performance and memory usage
Refactoring of cell object to eliminate data duplication and reduce memory
Changelog entry for refactored calculation engine
Refactor the cyclic reference stack as an object so that we can inject it into the logger constructor rather than having to pass it at every call
Merge branch 'develop' into calcEngine
Fixes to Advanced Value Binder for cell restructuring
Correct attachment of cells to cache collection as parent rather than worksheet
Method for moving cells in the cell cache without needing to access the cell itself

TODO Sqlite cell cache needs equivalent method for updating cache database rather than simply the indexed cache array
Merge branch 'calcEngine' of https://github.com/PHPOffice/PHPExcel into calcEngine
Add move cell method for SQLite.

TODO - modify SQLite3 to use parameterised statements consistently throughout
Merge branch 'develop' into calcEngine
Ensure that values returned get getCalculatedValue aren't array values, irrespective of the returnArrayAsType setting
Make consistent use of prepared statements
Merge branch 'develop' into calcEngine