Refactor to conform to WordPress Coding Standards

[greatislander]

I put in an hour this morning bringing the User Interface Options plugin into alignment with the current WordPress Coding Standards. This is still a work in progress. FYI @colinbdclark @jhung @waharnum. Note that this does not include any JavaScript localization setup (#8); I intend to tackle that separately.

So far, this PR:

• [x] Brings PHP code into alignment with wordpress-coding-standards/wpcs@2.1.0.
• [x] Prefixes function names to avoid conflicts (as recommended by WordPress).
• [x] Sets default options in a plugin activation hook to reduce calls to get_option().
• [x] Adds CSS and JS asset versions to support cache invalidation when the plugin is updated.
• [x] Adds explicit jQuery dependency to avoid issues if jQuery is loaded in the footer.

TODO:

• [ ] Review admin settings page against current best practices.
• [ ] Add localization of admin settings labels and interface text.
• [ ] Update readme to pass WordPress.org readme validation.

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