Look for _posts directory relative to cwd, not relative to tumblr.rb.

That's where we put it earlier, so that's where we'll find it now.
Fix truncation of overly long post names.

Delete the old truncate_post_name; it was called too late (if url rewriting
is enabled which it is by default), didn't run (it tried to use + to concat
a Fixnum onto a String), and even with those problems fixed, didn't actually
shorten the string enough to use as a pathname.

Instead, apply simple string truncation at the point we generate the slug,
which is used in the filename and is the part that could be unboundedly
long. I arbitrarily chose 200 as the maximum length; even shorter might be
better (really long slugs are just visually ugly); it might also be nicer
to truncate at a hyphen boundary.