Many tweaks for homepage

This improves many aspects of the homepage design.
-- Improved view on devices < 727 px wide
-- Improved structure of javascript
-- Fixes bug of display size between 727 px and 767 px
Wire up login and logout links
Make the logo a link to /
Improve the look of the signin process a little bit
Lock in rbenv ruby version at 1.9.3-p327

We can update to ruby 2.0 soon, if that suits us, but I don't feel like
installing it right now.
Add URL to posts

Let's store the original URL for each post
Skeleton of PostController

This is the beginning of being able to see the comments for a post, as
well as upvote and downvote it.
Post display page, including the idea of Comments

This wires up the post show page. It's a bit meager right now, but I
wanted to take a crack at the most interesting part -- the comment
system.

To this end I have found an interesting library 'awesome_nested_set'
which is way more interesting than just setting a parent_id on the
comments to refer to their parents. You still have to set a parent_id,
but it uses the IDs of comments in a clever way so it improves the
efficiency of some operations. Namely
-- you can select all descendants of a comment: one SQL query
-- deleting a comment and its descendants: one query
-- more benefits here:
  http://threebit.net/tutorials/nestedset/tutorial1.html
(The downside is that the SQL for inserting a comment is pretty
ridiculous. Not a lot of queries -- but one very complicated query.
Fortunately, we can assume it's well tested).