Fixed errors on settings page

[ashleyfae]

I fixed an undefined index error in the "Book cover size" field. You were referencing an array key that didn't exist in the defaults: $selected = $options['thumbnail'];. I'm not sure what you actually wanted to be the default there so I went with 'thumbnail'.

Then I fixed a few small other things:

1. You were doing like a double echo on the selected() functions, which resulted in some extra selected="selected" markup. I just set the third parameter to false in all instances of selected().
2. I escaped all the values.

[jazzsequence]

lgtm thanks!