Make plugin generator consistent with gemplugins

Should I also change locations of tasks and generators (move them into rails/ dir)? If you +1 this, be sure to take a look at #441 Add more support for gems frozen in the vendor directory too.

+1

Still the case,

ncr:~/dev/edge$ ./script/generate plugin foo
      create  vendor/plugins/foo/lib
      create  vendor/plugins/foo/tasks
      create  vendor/plugins/foo/test
      create  vendor/plugins/foo/README
      create  vendor/plugins/foo/MIT-LICENSE
      create  vendor/plugins/foo/Rakefile
      create  vendor/plugins/foo/init.rb
      create  vendor/plugins/foo/install.rb
      create  vendor/plugins/foo/uninstall.rb
      create  vendor/plugins/foo/lib/foo.rb
      create  vendor/plugins/foo/tasks/foo_tasks.rake
      create  vendor/plugins/foo/test/foo_test.rb