Update environment setup instructions

* Move Atom instructions earlier so we can reference them
  if user needs to modify ~/.bash_profile
* Update Homebrew installation instructions
  (they were outdated, and if you followed them,
  it would tell you what to do,
  but also to notify whoever gave you the instructions that they need
  to be updated) Also, explain why it needs root permissions
  (as I always find this suspect).
* Add instructions for how to fix the path if `brew doctor`
  complains about /usr/bin showing up before /usr/local/bin.
  This seems to happen frequently enough that we should discuss it
  (we should probably also have material on what the path is,
  why it matters, how to modify it, etc)
* Add instructions for how to start PG explicitly,
  and expand on instructions for how to have it always be on.
* Remove Postgres instructions about chowining shit.
  I'm almost certain this is what caused me to spend
  easily over a hundred times more time/effort fixing students'
  environments, than if they'd just `brew install postgresql`.
  (I checked that we don't need this by trying all these things
  on my sister's computer)
Merge branch 'master' of github.com:JumpstartLab/curriculum