Make zlib optional

[larowlan]

Q	A
Bug fix?	no
New feature?	yes

The composer.json currently has a dependency on ext-zlib, however this project is useful without it, if you use ZipCompressionMethod::STORED for all files, you can still do useful things like create archive, password protect them.

This change moves the decision about which format to automatically use behind a method on FileUtils that also does the extension loaded check