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

right now the description is not escaped, and it is rendered straight
to markdown on the competitions page. this will run javascript that is
saved in the database. FIX THIS NOW.