Additional Placeholder Class

I want to be able to apply an existing class to the placeholder.

My scenario is that I am using bootstrap wells for the styling of my items and I don't want to have to replicate the style from a well onto the .as-sortable-placeholder class. Instead, I want to be able to just apply the .well class to my placeholder.

This pull-request allows for `additionalPlaceholderClass` to be a defined on the options object passed to the as-sortable directive.

I have also updated the Readme to include documentation regarding how the placeholders are constructed, how they may be styled and how to use this additional class option.