Layout and styles

* Layout using Bootstrap v3
* Styles by @kcjpop
Add session support

Use cookie to manage a session id, but other session data are stored
in  a datastore. Currently ETS data store is supported as default.
The GIST client instance for each user is then kept in session.
Add gist's author on the sidebar

This completes the styles, closing #3 as well as closing #2 until
@kcjpop improves the UI.

Will need to change the way the comments are fetched, rendered
but for now, need to focus on other stuffs first.