Adding a tag generator to create asset tags.

This allows Assets to abstract the actual creation of tags.
s/Assets/AssetManager.

Refactoring to use TagGenerator and allow for groups of assets.
Simplifying TagGenerator by removing attributes option.
AssetManager can now load asset groups from config files.
Updating assets view extension.
WIP refactoring of AssetsController.
Big overhaul of assets docs.

The rumours are true - I'm finally attempting to add some good
documentation.
Small fixes to assets docs.
Updating AssetManager to be in line with docs.
Adding AssetsModule.
DatabaseFactory now creates Doctrine connections.
Updating DatabaseService.
Requiring doctrine dbal in composer.json.
Removing database stuff, to be replaced with DBAL and active doctrine.
Greatly simplifying AbstractMigration.
Refactoring MigrationRunner to use DBAL and Schema tools.
Merge branch 'assets'