Enhance keyboard navigation, bugfixes, options.

* Bug fixes:
 - Added check for next/previous stories to fix errors that were popping up when using keyboard navigation
 - Removed non-existant "h" keybind, as the help function did not exist.
 - Fixed bug where keybinds would stay active when submitting a new HN story, causing the page to navigate behind modal when typing.

* New Keybinds
 - Using j and k will now also browse up and down through comment pages, highlighting the comment you're actively reading.
![Screen Shot 2013-03-03 at 2 37 25 PM](https://f.cloud.github.com/assets/566763/214656/d1e92d84-8439-11e2-9204-1458932bfa1a.png)
 - Pressing "a" will now upvote the currently selected story or comment.
 - Pressing "l" will now open a story and related comments, each in new (unfocused) background tabs. I added this feature so that a user could browse the HN home page, opening stories & comments that they find interesting, and continue browsing. 
 To build this functionality, it was necessary to add a background page to create new tabs. Chrome content_scripts do not have access to chrome.tabs, only background/event pages can access or modify tabs. No extra permissions were needed for creating tabs.
 For more info on:
  Tabs: http://developer.chrome.com/extensions/tabs.html
  Messaging: https://developer.chrome.com/extensions/messaging.html
![Screen Shot 2013-03-03 at 2 16 24 PM](https://f.cloud.github.com/assets/566763/214655/9cdca6f2-8439-11e2-86b6-51cf4c0cb6a5.png)

* Options page
 - Disabling Hotkeys: I was reading through reviews on the Chrome Extension page, and a user mentioned that they loved the design but the keybinds were a dealbreaker.
 "Makes HN readable and lovely, but really needs the ability to disable the keyboard controls."
 I agree that having keybinds on permanently can cause issues with other plugins, so I added an options page. Unfortunately this required adding the permission to use Storage to the manifest.json file. I decided to go with chrome.storage instead of localStorage, to avoid having to use a background page to retrieve user preferences from the hn.js content script.
 For more into on chrome.storage:
  https://developer.chrome.com/extensions/storage.html

 If you'd prefer that I remove this options page from the pull request, please let me know  and I'll resubmit the pull request without it.
![Screen Shot 2013-03-03 at 2 29 15 PM](https://f.cloud.github.com/assets/566763/214657/efc09086-8439-11e2-9fc5-c924415f51ec.png)

![Screen Shot 2013-03-03 at 2 29 02 PM](https://f.cloud.github.com/assets/566763/214658/f6e11318-8439-11e2-9691-7d23ba389ab8.png)