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.

This addresses part of issue #773 (subissue 3).
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.

This fixes the rest of issue #773 (subissue 4).
Use Dir.pwd instead of "." as the source argument to Post.new, since
this has historically supplied an absolute path in this call.
Merge pull request #775 from metamatt/issue773

Address issue 773 (bugs preventing tumblr.rb from running)