Lots of style changes

* Switched to some more idiomatic haml and sass
* General stylesheet and markup cleanup (opted for more generic styling over extra stylesheet overrides)
* Fixed some invalid markup that led to strange rendering issues
* Reassesed bootstrap usage and tried to ensure proper structure
* Bumped the sponsor images to the natural size specified by meetup and ensured that the sponsor logos don't get cropped
* Added tooltips to sponsor and photos to convey more info
* Cleaned up and simplified styling in event listing
* Added RSVP button to event listing
* Other misc