In-memory implementation using SplFixedArray
Updated code and tests to support errors when reading XML nodes (useful when reading XML files used for attacks)
Removed LIBXML_NOENT option (which DOES substitute entities...)
Added test for Quadratic Blowup attack
Added proper support for booleans, dates, numbers, errors.
Added unescaping of the read string.
Fixed a bug when cells did not have any values => now returns empty string.
Added Sheet class for the XLSX reader that exposes basic sheet info, such as name or ID.
When retrieving the sheet data XML, added extra XML parsing to retrieve sheet data.
Added test
Escaped line feed characters in shared strings before processing them.
This makes every string remain on one single line and therefore allow
fast retrieval
Replaced usages of "\n" by PHP_EOL
Added test for multiline strings
