script/plugin no longer exists in Rails 3

Instead we should use "bundle exec rails plugin".  The
"-r rails-3" is to make sure that we get the Rails 3 version
of the theme, i.e. one that uses Rails.root instead of
RAILS_ROOT.  (Unfortunately, it doesn't work specifying an
annotated tag with "-r", so we use a branch name instead,
say, of "use-alaveteli-0.10".)
Bump ALAVETELI_VERSION to 0.10 (should be the first Rails 3 version)

One of the mechanisms by which the correct version of a theme
to use is chosen is by checking whether there's a tag in the
the theme repository that matches the ALAVETELI_VERSION.
Since the themes need to have code changes for Rails 3, it
makes sense to define a version number to be the first Rails 3
version so that we can work on this branch and have working
versions of themes installed.