[JRO] Header markup/semantics and supporting SASS

I'm a stickler for proper tag semantics and trying to be as sparse with the selectors as I can. So this commit outlines some ideas to make the markup a little more HTML5 friendly and to reflect what the actual content is we're marking up.

1. The header div can just be a header that's keyed off w/`#user-profile #content header` instead of using a verbose id selector like `#user-profile-header`. The ID itself is a smell because it contains (essentially) a parent ID in it (`#user-profile`) and the name of a tag that can be used instead of a div - `header`.
2. The information you're displaying next to the avatar is, visually, a list. So it should be marked up as such.
3. No need for the `user-profile-header-info` ID since it's not used anywhere in the stylesheet.
4. Nitpick: use `<strong>` instead of `<b>`. See: http://www.think-ink.net/html/bold.htm
5. Semi-nitpick: Using ID's instead of classes in CSS is generally frowned upon (it's something that's been hammered into me over the past year or so). For more info: http://screwlewse.com/2010/07/dont-use-id-selectors-in-css/ & http://csswizardry.com/2011/09/when-using-ids-can-be-a-pain-in-the-class/
[JRO] Updates to users controller

* Don't use find_by_* magic method as it will be deprecated in future Rails release. Instead use `where('…').first`
* Avoid unless if possible. Use `if @user.nil?`
* Temporarily hold nothing in @recent_monthly_posts until that method can be sped up
[JRO] Use P tag. Avoid double negative

* Instead of using a span, use a <p> as it more closely resembles what that chunk of text is - a paragraph.
* Avoid double-negative: `if presence of` instead of `unless absence of`. It's quicker for comprehension as you don't have to untangle the double negative.
[JRO] Markup cleanup. Let rails render collection

* Instead of applying a user-profile-recent-posts-th class for all of these TH's, just target the th's with a specific selector like `.user-profile thead th`. It keeps the markup more lean and allows for the same targeting of those elements. Applying descriptive and short classes to those th's will be helpful in the future if they need to be differently styled.
* Instead of looping through the @user.topics collection you can hand off that collection to rails and let it render partials on its own. The call to render I used to replace it with allows us to clean up the view.
Update to include @jayroh's changes
Extracted out the date formatting to a helper for re-use
Trimming the CSS and markup fat

- Refactored the th and td into more managable selectors
- Swapped out span for p selector
- Removed user post history graph