Parameterizing edx git repository location

* Creates global variable $platform_code_dir to store
  /opt/wwc/edx-platform, to replace the semihardcoded /opt/wwc/mitx that
  we had before.
* Plugs $platform_code_dir in a lot of different places
* Templatize sef's update script hack
Addressing jarv's concerns
Merge pull request #73 from edx/feature/jrbl/param_git_target

Parameterizing edx git repository location