Add factory models. Start implement api calls

* Add factory models for App and OpenDevelopUser
* Create 2 classes for single and list view of Sandbox
* Create file logic in sandbox app. This file will implement the
  backend part of the sandboxes
GET /api/sandbox, GET /api/sandbox/id
Fix PEP8 styling
api: Add more tests

Add tests to check error return codes from the oauth decorator
api: Create POST /sandbox api call and tests

This is only a dummy api call so far.
The backend is mocked with patch
Create bucket for sandbox

Create celery job to get the files from the request and put them
in the directory /etc/opendevelop/buckets/sandbox_id. The POST request
returs the sandbox_id and the process is asynchromous.
NEXT STEP: Implement call to docker api
Mostly code formatting and documenting

1. Removed abbreviations like "sbx" instead of sandbox that were causing
code inconsistency.
2. Documented sandbox.views module and all of its classes and methods
3. sandbox_to_dict is now a method of the SandBox model instead of a
standalone function
4. Removed "content={...}" from JsonResponse constructing, since content
is the first argument and does not need to be called by name
Fixed identation
Tests succeed

* Moved creation of SandBox back to SandBoxListView
* Missing image at sandbox creation returns 400 instead of 404. Test
modified to comply with this.
Added docker.py to requirements.txt
Removed status from sandboxes.logic.create

Removed status from invocation of sandboxes.logic.create since it was
initialized to SandBox.CREATED, which is the default value for that
field.
Added South to INSTALLED_APPS
Added South migrations

1. Added migration files for `sandboxes` app
2. Added docker_server field to SandBox model
Fixes in sandbox.logic

* When creating a sandbox, a new Image object is created by the given
  image_slug
* When creating a sandbox, a random DockerServer is being selected for
  the sandbox to run
Started SandBox tests
Started imaged migrations

* Default value for Image.url is None now
Added images migrations
Added __unicode__ methods to models

Added to following models:
  * api.models.App
  * common.models.DockerServer
  * images.models.Image (from previous commit)
Started migrations for Sandboxes
Added __unicode__ method to SandBox