NEW (#8): Ability to apply grid field capability to pages via DataExtension

[patricknelson]

Following up on #8. The basic approach here is to migrate core functionality to a trait so that this module's functionality can be applied via either:

1. Standard PHP class extension
2. Using SilverStripe's extensions (invented before PHP's native trait implementation)

In this case it does technically change the minimum requirements to support trait so that we can share our own functionality between two totally different classes (Page and DataExtensions) for a similar reason; to keep things DRY. As a result I've had to change the following things:

• Migrated core code from GridFieldPage to a new trait and including via require_once since SilverStripe 3.x's autoloader doesn't support traits (probably due to their PHP 5.3 compatibility).
• Updated composer.json requirements to reflect the new PHP requirements
• Updated README.md to call this out as well

Maybe this change would bump the current module version up to 2.0 instead of 1.0 since technically this is a breaking change in case anyone out there is somehow still on PHP 5.3? 🤔