add rakefile with test and release tasks; add version file

This rakefile is adapted from my standard one and adds several useful features (run `rake --tasks` for an overview):

* `rake test` and variants: run tests regardless of cwd

* `rake release:log`: see all log messages since last release

* `rake release:diff`: see diff since last release

* `rake release`: do all the steps to release a gem: check that VERSION has been changed, commit, tag, and push, let the user edit the commit message (which will also be visible in the releases tab of the github page), build and push the gem.

There are lots of popular gem release management tools, but this one is simple enough to copy and paste between projects and edit as needed.