bootstrap select implementation

Implementation of select and multi-select with http://silviomoreto.github.io/bootstrap-select/ js plugin
Let's make this:
````java
add(newBootstrapSelectBehavior(config));
````
with 
````java
protected BootstrapSelectBehavior newBootstrapSelectBehavior(BootstrapSelectConfig config) {
  return new BootstrapSelectBehavior(config);
}
````

This way if anyone needs to do something custom (s)he can override it.
This will solve the "problem" with the static `omitCss` variable.