Add module for CVE-2013-6955

Auxiliary module for Synology DiskStation Manager (DMS) SLICEUPLOAD
vulnerability, which allows unauthenticated remote command execution
under root privileges.
Use 'unless' instead of 'if not'
Msf::Exploit::Remote::HttpClient already provides 'peer'
Turn Auxiliary module into Exploit module
Do final (minor) cleanup
Revert gsub!
Merge pull request #2 from jvazquez-r7/review_2787

Do final (minor) cleanup
Do send_request_cgi final clean up
Land #2787, @mwulftange's exploit for CVE-2013-6955