Comparison operators on strings are usually case insensitive

Excel, Gnumeric and Google Spreadsheet are case insensitive, so the
default behavior of PHPExcel is modified accordingly.

However OpenOffice is case sensitive and is also supported via the
compatibility mode of PHPExcel.

Fixes #31
ISTEXT() return wrong result if referencing an empty but formatted cell

ISTEXT should always return FALSE for empty cells, however PHPExcel returns
TRUE if the cell is formatted. This can be reproduced in Excel by choosing
formatting category "Text" for cell A1, and then in cell B1 input the
formula '=ISTEXT(A1)'. B1 will display FALSE, but PHPExcel will return TRUE.

This patch fix the NULL value being incorrectly cast to an empty string, and
thus eliminating ISTEXT() issue (and probably several others).
Merge pull request #273 from PowerKiKi/issue_is_text

ISTEXT() return wrong result if referencing an empty but formatted cell
Bugfix: (PowerKiKi) Work Item GH-273 - ISTEXT() return wrong result if referencing an empty but formatted cell
Merge pull request #270 from PowerKiKi/issue_31

Binary comparaison of strings are case insensitive
Bugfix: (PowerKiKi) Work Item GH-270/GH-31 - Binary comparison of strings are case insensitive
Convert properties to string in OOCalc reader
Feature: (trvrnrth) Work Item GH-261 - Add support for reading protected (RC4 encrypted) .xls files (tweaked for PHP 5.2 compatibility)
Merge pull request #276 from infojunkie/OOCalcReaderXML

Convert properties to string in OOCalc reader
General: (infojunkie) Work Item GH-276 - Convert properties to string in OOCalc reader
Mods to handle encrypted files on both 32-bit and 64-bit versions of PHP - currently only tested on 32-bit Windows
Update Calculation.php

PHPExcel Fatal error: Call to a member function cellExists() line: 3327
Bugfix: Work Item GH-275 - Insert New Row/Column Before is not correctly updating formula references
Bugfix: Work Item GH-275 - Insert New Row/Column Before is not correctly updating formula references
Minor fix to chart layout
Bugfix: Work Item GH-257 - Passing an array of cells to _generateRow() in the HTML/PDF Writer causes caching problems with last cell in the range
Bugfix: Work Item GH-193 - Fix to empty worksheet garbage collection when using cell caching
Bugfix: (Jazzo) Work Item GH-248 - Excel2007 does not correctly mark rows as hidden
Bugfix: (Roy Shahbazian) Work Item GH-299 - Fixed typo in Chart/Layout set/getYMode()
Improve fluent interface in charting methods