renamed indexdir vars to index_dir
made index size sim more forgiving
total overhaul of search engine

* Eliminating duplicate postings earlier on to speed up indexing and
  simplify the code a little; inadvertently fixed an undetected bug
  that was allowing duplicate postings through.  (Probably the same
  posting ended up in different segments previously and then didn't
  get dup-squashed when merged.)  Another surprising effect of this
  was that my example 117MB dataset no longer required any merging,
  allowing me to index it in 6m10s instead of the previous 20m.  Now I
  need a bigger example dataset to test on so the merge code gets
  exercised.  I'm fixing this by reducing maximum segment size.
* Removed comments that are now covered by the text.
* Switched to using a Path class for almost all pathnames in the code.
  It provides a little syntactic sugar, that's all.
* Eliminated some unused variables (or rather made them be _)
* Factored out tuple-file reading and writing functionality.
* Added some directory metadata indexing code in preparation for
  incremental index updates.
* Added line numbers to the grep output format so you can use it in
  Emacs grep-mode.
more updates to search-engine text

I added a section explaining query evaluation.  It could benefit from
being more concrete.  The code probably needs to be changed quite a
bit still; it's about 37 lines.