Fix IATA code for Aegean

A3 not AG lol oops
Add Aegean graphic
Remove reference to chosen stylesheet

Not using it anymore
Add usage, contribute pages
Add header to about page
Add SWISS airline graphic
Add SWISS earning info with United, Aegean
Modify Brussels Airlines graphic

Moved it up a bit so that it looks more in line with the rest of the
graphics.
Update table display on homepage

Now uses an airline grid for choosing airlines. Closes #3. Still only a
prototype.
Fix bug in add_mileage_multiplier