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
Using ansible instead of ssh
Seperating out Xqueue and xqueue_consumer restarts
Apache LMS template listen on all ports
resolves #1624
Turning off basic auth for segement.io
Merge pull request #1627 from edx/origin/han/nginx-edxapp

Turning off basic auth for segement.io
Merge pull request #1625 from edx/cg/fix_apache_ports

Apache LMS template listen on all ports
replace geturl with a shell out to curl - to fix broken certificate management
Merge pull request #1628 from edx/derf/replace_get_url_with_curl

replace geturl with a shell out to curl - to fix broken certificate management
update comments
Merge pull request #1629 from edx/derf/replace_get_url_with_curl

update comments
EDXAPP_MONGO_PASS is not a valid variable.
Merge pull request #1630 from edx/feanil/fix_connect_sandbox

EDXAPP_MONGO_PASS is not a valid variable.
replace geturl with a shell out to curl - to fix broken certificate management
Merge pull request #1631 from edx/han/release-fix-geturl

replace geturl with a shell out to curl - to fix broken certificate mana...