Fix #19.  Update 'bootstrap-combobox.js' and step3 HTML/JS/CSS.

Add 'prevententer' option to 'bootstrap-combobox.js' to prevent enter button
from submitting form.  Move change event triggers in 'bootstrap-combobox.js'
so 'combobox-selected' class is added before change event is triggered.
Automatically add selection to pillbox and remove 'Add' button.