Fix rakefile error and use bundler-style gemspec, etc.

Hi @jystewart

When checking out this project I got:
```
$ rake -T
rake aborted!
ERROR: 'rake/rdoctask' is obsolete and no longer supported. Use 'rdoc/task' (available in RDoc 2.4.2+) instead.
/home/michael/projects/xss_terminate/Rakefile:3:in `<top (required)>'
/home/michael/.rvm/gems/ruby-1.9.3-p448/bin/ruby_noexec_wrapper:14:in `eval'
/home/michael/.rvm/gems/ruby-1.9.3-p448/bin/ruby_noexec_wrapper:14:in `<main>'
(See full trace by running task with --trace)
```

It seems the whole rakefile, etc. is out of date, and personally I find jeweller adds yet another dependency when bundler is almost universal now so I've generated a new project with bundler and used those files as the basis to fix the rakefile, add a Gemfile, etc.

This also fixes an annoying gemspec error you get when bundling this gem.

If you have any questions please let me know.