DRAFT: 3.0.0 Release Branch

[cameronterry]

This PR is the 3.0.0 release branch, which will include a number of significant changes to the Dark Matter plugin.

Release Date: T.B.D.

This PR will contain the following changes:

• Change plugin name from "Dark Matter" to "Dark Matter Plugin".
• Utilising a PSR-4 autoloader (included in this branch).
• Switched domain mapping to use the new autoloader: https://github.com/cameronterry/dark-matter/pull/103
• Custom Redux store for domains; [PR T.B.D.]
• New Single-Sign On module for; https://github.com/cameronterry/dark-matter/pull/109
• Full page caching, a new feature: https://github.com/cameronterry/dark-matter/pull/86