feature: adding date variables, including a custom medium date

## Ready State: (Yes)

This pull request is related to the following JIRA tickets: [LDC-230 Create panelized news article display page](https://imagex.atlassian.net/browse/LDC-230).

## Requirements & Dependencies

## Deployment Instructions

* Merge code.
* Enable module `imagex_date`: `drush en imagex_date`
* Clear caches with `drush cc all`

## Usage/Test Instructions

* the medium date format should be customized. I assigned it after creating a new date formatter. Unfortunately date formatters can't be exported in features yet, but the actual formatter is assigned to the medium date type.