Reduce classname conflict potential by introducing BEM or something similar

[patricekaufmann]

The class names are very generic at the moment. Classes like `select-item, search, options, panel-content, select-panel, dropdown-heading, dropdown-content' etc. are not very specific and could lead to conflicts in projects with a larger scope.

Class names like rmsc__options would help encapsulating the classes. It would also help avoiding nested selectors like .rmsc .dropdown-content .panel-content as this only costs performance when evaluating css; rmsc__panel-content would probably be sufficient.

What do you think?