Set sw=0 sts=-1 instead of setting them both to the value of 'ts'.

Should be functionally equivalent, but avoids problems for people who
set softtabstop=-1 in their .vimrc and expect the value to stick.
Merge pull request #4 from dlitz/fix-softtabstop

Set sw=0 sts=-1 instead of setting them both to the value of 'ts'.