Fix wrong options in 7zip command line.

The 'e' subcommand, that was used by the unpack method
in previous revision, extracts all files in a single directory.
It is replaced by the 'x' subcommand in order to extract files with full paths.

Additionally, the -uy switch is replaced by -y as -u does not seem
a valid switch with respect to 7zip doc
(see http://sevenzip.sourceforge.jp/chm/cmdline/commands/extract_full.htm).