Add support for lzma2 compression.

This patch adds support for lzma2 compression provided by php-xz extension https://github.com/payden/php-xz.
I copy & pasted code and changed file reading and writing methods.

I've checked mostly used methods (`listContent(), addModify(), createModify()`) with **.tar.xz files**.
They work well.

Also I can update docs and add them too.