Rationalize requirements files and adjust one-time setup scripts to addr...

Rationalize requirements files and adjust one-time setup scripts to address issue #280.

1. The list of requirements files is shortened.
1. The dev_setup.* files are modified to match the instructions for getting started and getting the source code on the wiki. In particular the database setup is no longer done in the setup scripts.
1. Changed `.gitignore` because some items were not ignored. I don't understand the use of the `**/**` pattern in many places.

Notes for Will about https://github.com/codalab/codalab/wiki/Dev:-Get-the-Source-Code:

* Step 4, Windows: `venv\Scripts\activate` (instead of *.ps1). Ditto in Step 11. On Windows this is a no-op as Step 3 leaves you in the virtual env.
* Remove Step 5. Current instructions are incomplete; better remove the step which is not necessary at this stage. Plus, this part of the setup will change (removing dependency on RabbitMQ).
* Step 6: `cd codalab` should be sufficient 
* Step 6: both Win and Linux
    * Remove the step to generate config. Not needed here.
    * Add: `python manage.py validate` just after `cd codalab`. Validation should finish stating no errors were found.
* Step 9: `python manage.py runserver` is good enough to start with.