Updated the "rake list" command to print tasks with descriptions.

Hi!

Just wanted to suggest a feature for the `rake list` command so that it can print the tasks with descriptions as well. 

I made sure that my change retains the previous behavior of the `rake list` command. And I also needed that small hack at the top of the Rakefile for rake with versions >= 0.9.2.2 ([here's more information](http://stackoverflow.com/questions/825748/how-do-i-pass-command-line-arguments-to-a-rake-task)). 

Here's how `rake list` looks after the change:
```
Tasks: clean, copydot, deploy, gen_deploy, generate, install, integrate, isolate, new_page, new_post, preview, push, rsync, set_root_dir, setup_github_pages, update_source, update_style, watch
(type 'rake list[true]' for more details)
```

And how it looks after typing `rake list[true]`
```
Tasks: 
  'clean' - Clean out caches: .pygments-cache, .gist-cache, .sass-cache
  'copydot' - copy dot files for deployment
  'deploy' - Default deploy task
  'gen_deploy' - Generate website and deploy
  'generate' - Generate jekyll site
  'install' - Initial setup for Octopress: copies the default theme into the path of Jekyll's generator. Rake install defaults to rake install[classic] to install a different theme run rake install[some_theme_name]
  'integrate' - Move all stashed posts back into the posts directory, ready for site generation.
  'isolate' - Move all other posts than the one currently being worked on to a temporary stash location (stash) so regenerating the site happens much more quickly.
  'new_page' - Create a new page in source/(filename)/index.markdown
  'new_post' - Begin a new post in source/_posts
  'preview' - preview the site in a web browser
  'push' - deploy public directory to github pages
  'rsync' - Deploy website via rsync
  'set_root_dir' - Update configurations to support publishing to root or sub directory
  'setup_github_pages' - Set up _deploy folder and deploy branch for Github Pages deployment
  'update_source' - Move source to source.old, install source theme updates, replace source/_includes/navigation.html with source.old's navigation
  'update_style' - Move sass to sass.old, install sass theme updates, replace sass/custom with sass.old/custom
  'watch' - Watch the site and regenerate when it changes
```