clean up build instructions, and remove macports instructions...

* Macports build instructions were incomplete. The main blocker was that
packages/darwin_app/ is hard-coded to use
homebrew paths instead of Macports or Fink paths. That could be fixed,
but there are bigger problems:
a) having two build environments splits the developer knowledge-base in
half and potentially doubles the number of build issues
b) Macports requires full XCode which is multi-gigs of download and
probably hours more of installation time. In fact I ran out of space on
my OSX VM just from the App Store trying to update XCode!
7 jobs for master in 48 minutes and 6 seconds (queued for 691 minutes and 25 seconds)