persist cookies for 48 hours

I chose 48 hours to encourage competitions to be short. Most users
should not have to log in more than once per competition. Note that this
involves creating a SECRET_TOKEN in env_keys.yml. A simple way to do
this is using the command

`ruby -rsecurerandom -e "puts SecureRandom.hex(32)"`
update the README for project updates

add new installation instructions:
  install MongoDB (which was left out of the original instructions)
  creating a SECRET_TOKEN

remove front end description that claims the project uses bootstrap
use valid? instead of save! to test competitions

save! will try and hit the database, and therefore is slower
validate a team cannot exceed membership

teams have a TEAM_LIMIT that enforces how many members they can have,
but it wasn't actually enforcing itself. it would only report whether or
not a team was full but members could still be added.
remove extraneous time_helper file

this file was accidently kept because the filename in vim wasn't
changed. it should never have been committed.