New Plugin: Admin Styles

Adds a small variety of custom styles to the Admin-plugin, by injecting CSS-files which override the default. All styles are based on the default style for the Admin-plugin, but do not altar it. A preview of the available styles can be found in the README.

The pre-release is available from GitHub, and I would appreciate any testing and reports of issues, inconsistencies or oddities through the issue-tracker.

.gitignore is not friendly, so rather see the most up-to-date release for package.