Adding titles to link posts

*adding ability for users to call their links something else besides the URL.
*added title column to link_posts table
*made url title appear on home page rather than url
*Centering link to see all posts
Merge pull request #17 from DK-2K14/ks-link-titles

Adding titles to link posts
adding Gemfile.lock
Adding polymorphic comments/commentable

*allow user to comment on posts or comments