Adds an API to create a sub-account.

This functionality already exists through the UI, but is currently missing
from the API. This feature is useful when populating a local installation
with test data through the API, and was also requested at
    https://help.instructure.com/entries/22875265-Add-REST-API-for-Adding-Sub-Accounts

The changes adds a new route to POST to 'accounts/:account_id/sub_accounts',
the logic to handle a create request and a spec test to verify its behavior.

I have run the new spec locally (along with the other tests in
accounts_api_spec.rb), but have been unable to run the full test suite due
to the issues reported at
    https://groups.google.com/forum/#!topic/canvas-lms-users/TcI0c6pTAUo
Update to Add Sub-Account API

Modified the change to account for Cody's comments:
 - moved the routing and documentation to the sub_account controller.
 - removed the time zone parameter from documentation and spec.
 - used account_json to minimize the data returned.

The last change required modification to the sub-account display Javascript
to accept the modified return value from the create handler.
Update to Add Sub-Account API

Tagged the sub account controller for the API doc generator.
Update to Add Sub-Account API

Minor fix to make the sub-account update controller return the same format
as the create controller, since both cases are rendered using the same
Javascript code.
Update to Add Sub-Account API

Removed the parent_account_id parameter from the REST API requirements.
If the parent_account_id field is supplied (ie, the request comes from a
view) we use that, otherwise we use the account_id passed in the URL.
Merge pull request #372 from mcgachey/master

Add an API to create a sub-account.