Set permissions on media files to 0640

By default the permissions will be as they were for the Tempfile, which
is 0600.  This means that other users on the same repo won't be able to
get the media files.

This commit adds group read permissions to media files.  Group write
permissions are not required as the file is identified by its contents
anyway, so it should never need to be modified.
Remove rdoc references
auto upload on clean. auto download on smudge.
fixed passthrough bug
show only first 8 chars of sha on clean/smudge
added smudge pass thru stderr print to see when it happens
use ruby built in scp / ssh
added scp/ssh port options. updated gemspec to include scp/ssh
removed clean/smudge debug output
fixed error when %f is not specified in the .gitconfig media filter section
catch Net::SCP exceptions and fail gracefully
set binary mode for temporary file to fix Windows bug
standardized and improved print output
updated gemspec
added list option (placeholder)