We had a bug where mixins weren't being applied before `load_from_xml`
was called. This meant that not all of the fields were being loaded
correctly. To fix it, we used the mixoligist from the runtime to apply
the mixins earlier in the process. However, that caused the mixins to be
applied twice.

The included fixes to xblock resolved the multiply-applied mixins, and
the fixes to the parsing code make it simpler to understand, and add
some unit tests of the parsing to boot.

Also added datadog application to commom django apps and set it to use
a local statsd server by default.

* Make currency a property of the Order (for validation purposes)
* Remove null=True from Char fields
* Use InheritanceManager for subclassing OrderItem
* Change VerifiedCertificate to better handle some new use cases
* Cleaned out old migrations
* Tests!

* Make currency a property of the Order (for validation purposes)
* Remove null=True from Char fields
* Use InheritanceManager for subclassing OrderItem
* Change VerifiedCertificate to better handle some new use cases
* Cleaned out old migrations
* Tests!

    Solves an issue with using git > 1.8.1 for github based pip requirements.

Two endpoints are intended to be used by AJAX calls from the web UI with an
authenticated user. When enabled, the value of the preference is an encryption
of the username, to be used as a token for one-click unsubscribe links. The
third endpoint is the target of unsubscribe links, which displays a page with
an appropriate message to the user.

The new API uses Django REST Framework. For now, it is designed specifically
to support the use cases required by the forum digest notifier (not yet built),
with a goal of making it more generally useful over time.

Replacing the get_request_method function everywhere

The most common error is Amazon SES having blacklisted an email address.
However, we handle any error being raised during user.email_user.

Fixes JIRA LMS-131

scipy can go back into base.  MySQL-python really does have to be installed after distribute has been updated by base.txt, I tested it.

Move Mysql and scipi to requirements/post.txt, because they require the contents of requirements.txt to be completely installed