Sirirus github.io redesign

New front-end, largely based on designs from Austin Wells in UX (approved by Jon Moore) handed off at the beginning of the summer. 

- Takes full advantage of Jekyll, using header/footer/includes.
- Design didn't contain Overview content, so I incorporated my own design for that into the single-page structure.
- Refactoring of markup and styles will be done in future commits unless something egregious is found.
- Additional enhancements can be done in future commits (i.e. additional pages or content, search, design tweaks, etc).