ubuntu-install.sh installs and runs spork and edx client in screens

- Runs pre-install, clone, test for:
  - saasbook/rag `Spring2014 branch`
  - saasbook/hw `Spring2014 branch`
  - saasbook/rottenpotatoes `hw3_solution branch`
- Runs spork process and the rag/run_edx_client loop in scripted named screen sessions Spork and Edx-client.
- Pivotal item Make a deploy package for rolling out rag by maintainers:
  - https://www.pivotaltracker.com/story/show/64134482 
- Currently requires manual edit for branch, does not run unattended, and requires creds for github private repos.

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- No way to automate the get of the autograder.yml and config file, or edit of them?
- Meanwhile, move all user interaction to the beginning:
  - sudo
  - the reminder of copying config files and editing them
  - Add read -p for GH_USER and GH_PASS, use in github login later.
  - Add read -p for branches, so user doesn't have to edit ubuntu-install.sh at all.
- Show config summary  for review and confirmation from user before launch, include at end too.
- Re-add the .screenrc part and source it.
- Echo git prompt into profile and source it.
- Ensure default scrollback is enough on screens or set it with -h 9999