Update core/models/file_list.php for windows compatibility

exportArchive() now using ZipArchive class instead of a linux command in exec(). We can now export images in Sample Content Generator package on Windows servers.
Update file_list.php

Additional check to make sure the ZipArchive extension is loaded and revert to the old behaviour if it isn't. Added a try-catch in case they both fail. Thanks Remo!
Update file_list.php

Fixed a mistake: the $filenames array was not built before the exec(). Sorry!
Update file_list.php

Replaced die() with Exception.
Update file_list.php

Removed redundant sprintf(). Thanks mlocati! Simplified the string concatenation in exec(). Also brought some curly braces back because some people really like them :\
Update file_list.php

Reduced the number of calls to $f->getPath() for simplicity and tidied up the foreach loops
Merge pull request #1024 from VahanPWNS/patch-1

Update core/models/file_list.php for windows compatibility