[ts paths] Fix bug with relative paths

If a non-existant relative path not prepended with ./ was given to
ts.create(), then it would fail as it couldn't determine the dirname to
create.
[ts paths] Better striping of trailing slashes

Trailing slashes fuck up basename and dirname, so we remove them if
they exist. This removes the literal '/', and uses os.pathsep, and also
makes removing trailing slashes the first thing that happens when
working with either ts.create or the ts.path property.