clean up build instructions, and remove macports instructions...
* Macports build instructions were incomplete. The main blocker was that packages/darwin_app/embed-MacOSX-dependencies.sh 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!