Prefix functions, space usage, add_sizes via hook

Some small tweaks:
- Function names are prefixed with `tevkori_` to [prevent conflicts](http://nacin.com/2010/05/11/in-wordpress-prefix-everything/).
- Added whitespace to better conform with the [WP coding standards](http://make.wordpress.org/core/handbook/coding-standards/php/#space-usage).
- Additional image sizes are added via a function which loads on the ‘plugins_loaded' hook.
- Removed [closing php](http://make.wordpress.org/core/handbook/coding-standards/php/#remove-trailing-spaces) tag.