Update CSS load order, add .has-warning for one of the button groups.
Allow enabling/disabling Select2 elements via checkboxes on the example pages.
Add examples for disabled Select2 inputs, fix disabled styles for multi Select2.
Revert border-left inherited from Select2's CSS to prevent the arrow from jumping when switching from disabled to enabled state and vice versa.
Minor improvements:

 * use reset-filter mixin
 * improve inline doc
 * fix border-top-color for .select2-drop-auto-width and -drop-above-active
 * inherit BS 3's default "Alert"-style for .select2-searching