Bootstrap style <select>s and optional better renderers for relations

The changeset includes:
* properly styled `<select>`s  with bootstrap3 (by adding class `form-control`)
* Rich select-boxes with [bootstrap-select](http://silviomoreto.github.io/bootstrap-select/3/)
* Two pane multi-select renderer (usedful for ManyToMany relations) [Bootstrap Dual Listbox](http://www.virtuosoft.eu/code/bootstrap-duallistbox/)
* Also several ManyToMany/ManyToOne relations added to scaffold (demonstrating the renderers)