Add a check for a compressed snap data

Snap data is compressed when a video is sent with an overlay (text, drawings or filters)
 - Check for compression
 - unCompress
 - Return both files as an array with keys (overlay, and m4v).
Add methods to support compressed snap data.

isCompressed: returns True or False
unCompress:   returns False or an Array of files with filenames as key, and content as values.
zip_open only likes files

zip_open fails with file content within a variable. So must write to a temporary file.
Added exit, for failing to write to own folder. Any seasoned contributor, do change if there are better ways to handle this.
All php variables are the same.

There is no difference between Array and String in PHP. All use $ as prefix.
Updating comments
Merge pull request #25 from hisham-albeik/identify_compression

Identify compression