Add `scw _userdata SERVER KEY=@path/to/file` support "à-la-curl"

Expected result: 

``` console
$  cat <<EOF > test.txt
heredoc> first line
heredoc> second line
heredoc> third line
heredoc> EOF
$  scw _userdata alpine test=@test.txt
test
$  scw _userdata alpine test
first line
second line
thirt line
$
```

See http://curl.haxx.se/docs/manpage.html#-d
