include (empty) spnet/sessions in repository

I had one last problem with a new spnet install:  The directory spnet/sessions is empty and therefore not included in the repository (git only tracks *files*).  When the server starts up, it fails because the sessions directory does not exist.

An easy fix is to include a .gitignore file (or any other file) in the directory, and commit that file.  Since we probably will never want any of the contents of spnet/sessions to be in the repository, I've set .gitignore to ignore everything in the directory; this could be changed if there *is* something that should be in the repository there.

c.f.  http://stackoverflow.com/questions/115983/how-do-i-add-an-empty-directory-to-a-git-repository