Enhanced compability with 7z on Cygwin

On some systems, following message keeps appearing for unknown reason:

>Can't load '/usr/lib/p7zip/Codecs/.keep-p7zip' (Permission denied)

...and yet the file is unpacked correctly. Prior to this change, this message
kept appearing in every downloaded subtitles. This commit fixes compability
with such systems by piping that message to /dev/null.
7z doesn't need to be in /usr/bin/ anymore
Added checks for dependancies