Add namespace. WPCS Cleanup Part 1.

[ronalfy]

Resolves #76

This PR adds namespacing to the plugin, and does some initial WPCS cleanup (Part 1). This is Part 1 as I left most of the core code of the plugin intact and just did some whitespace formatting using WP Coding Standards. There is still much left, but for the sake of this PR and your sanity, I've left that for a Part 2, which will require more intense testing.

1. Namespace for the plugin as proposed: MToensing\SimpleTOC
2. Package for the plugin: simpletoc
3. Namespace and do WPCS cleanup for file: simpletoc-admin-settings.php
4. Moving ID class to file: simpletoc-class-headline-ids.php
5. Plugin file plugin.php has been namespaced, with some initial WPCS formatting.
6. File phpcs.xml.dist added to enforce coding standards in IDE.

Since the plugin is namespaced, you can now worry less about possible naming collisions with other plugins. The structure has been kept simple as to not re-architect the plugin or mess with build steps.

[mtoensing]

Thank you very much!