Add ability to specify custom python requirements.

This lets you specify additional python packages to be installed
into the edxapp virtual environment.

The `EDXAPP_EXTRA_REQUIREMENTS` should be a list of dict objects
with `name`, `version` (optional) and `extra_args` (optional) keys,
that correspond to the options of ansible's pip module.

Example:

    EDXAPP_EXTRA_REQUIREMENTS:
      - name: mypackage
        version: 1.0.1
      - name: git+https://git.myproject.org/MyProject#egg=MyProject
adding libfreetype6-dev
Merge pull request #1540 from edx/han/libfreetype6-dev

adding libfreetype6-dev to release
New base AMI, updating AMIs for Ubuntu
Merge pull request #1563 from edx/han/release-sandbox-ami

New base AMI, updating AMIs for Ubuntu
Use boto 2.32.1
make jump box specifiable and overridable
Upgrading S3 pluging to 0.6
OPENEDX_RELEASE doesn't work properly with variable inheritance in ansible
Merge pull request #1607 from edx/fix-openedx-release-vars

OPENEDX_RELEASE doesn't work properly with variable inheritance in ansible
Adding basic exemption
Add New settings to the env config.
Merge pull request #1609 from edx/e0d/notifier-vhost

Adding basic exemption
Correct the name of the ansible vars.
Merge pull request #1610 from edx/feanil/social_footer

Add New settings to the env config.
Create course data dir
Merge pull request #1611 from edx/create-course-data-dir

Create course data dir
Cluster all to one machine.

Alsa, add connection logging to rabbitmq.
Get rid of commented task.
Merge pull request #1608 from edx/feanil/common_cluster_updates

Feanil/common cluster updates