API Filter by date created for files

This PR incorporates a date filter for assets, similar to the filter used by the sitetree filter.

Since the column shown in the grid in AssetAdmin is the `Created` column, this is the same column the filter acts on. The SiteTree filter however acts on the `LastModified` column.

The behat test cases rely on an extension to behat to allow the modified date of staging files to be specified. *Do not merge before silverstripe-labs/silverstripe-behat-extension#32*


ref: CWPBUG-144