Style seperation

Separated mandatory style (*which is needed for drag-drop to function*), and the optional styles (*which can and should be different for each application*).
This creates two files: **ng-sortable.css** and **ng-sortable.style.css**.    the second one is purpusly **not** included in bower main,  so wiredep (and alike) don't force your style into our app..  that is now optional.


Why?     otherwise we need to add styles to **override** yours (*for example: border-radius, background color, etc...*).   Better we **add** styles as needed, rather then override what we don't.   Much cleaner.