Add ext-xmlwriter as a Composer requirement

XMLWriter is surprisingly a different extension from xml; and so is not installed on some systems (such as Gentoo).  This commit enforces it to be installed, since it's necessary for the functionality of PHPExcel.
Double quote support for SUMIF() condition

SUMIF() condition can have double quote, such as '=SUMIF(A2:A4,">""",B2:B4)'.
This formula purpose is to compare the character double quote (").

In our previous patch (commit f1a1f525) we wrongly assumed that
PHPExcel_Calculation_MathTrig::SUMIF() expected the condition to escaped ('>""'),
but this is actually not the case in real use of PHPExcel, so the unit tests were
modified accordingly to use non-escaped condition ('>"').
Extend headers to handle IE 9 problems and IE/SSL caching problems
Bugfix: Work item CP20703 - Calculation engine incorrectly evaluates empty cells as #VALUE
Disabled libxml external entity loading by default.

If you want to allow loading external entity references in XML, call PHPExcel_Settings::setLibXmlLoaderOptions(0).
Merge pull request #322 from maartenba/develop

Disabled libxml external entity loading by default.
Updated documentation for XXE injection in readers
Warning in security docs
AS we're using simpleXML for xml reading still, we need to use libxml_disable_entity_loader(true); for XXE security patch
When libxmlloader options are teh default values, disable the entity loader as well. CVE-2014-2054 by MITRE
Merge pull request #323 from maartenba/develop

When libxmlloader options are the default values, disable the entity loader as well. CVE-2014-2054 by MITRE
Update security section in Reader documentation
Merge branch 'develop' of https://github.com/PHPOffice/PHPExcel into develop
Hopefully final tweak for XXE to prevent displaying errors if SimpleXML isn't available
Added CVE reference to changelog
Added XXE protection to HTML Reader
Bugfix: Work item CP20760 - Formula references to cell on another sheet in ODS files
Bugfix: Work item GH321, GH158, CP17824 - LibreOffice created XLSX files results in an empty file.
Bugfix: Work item CP20760 - Formula references to cell on another sheet in ODS files
Merge pull request #305 from PowerKiKi/fix_SUMIF_condition_including_double_quote

Double quote support for SUMIF() condition
Merge pull request #297 from navarr/composer-xml

Add ext-xmlwriter as a Composer requirement