make the competition date more visible

the dates are perhaps the most important piece of information about a
cmpetition, so they should be easy to find and visually pop.
add z-index the main nav

since the nav is positioned absolutly and the competition page titles
are floating left it wasn't displaying properly.
flesh out documention for Competition class

I like the format I used for the Team documentation. This helps me see
what is defined by ActiveRecord based on the db schema.
save and display descriptions for competitions

the description is saved directly into the database. when displaying, it
is escaped and rendered in markdown. look into escaping it before saving
and then reversing the escaping in order to easily edit.