Theming and label options

The functionality seems to work pretty good even filtering with entity reference view. Here are some thoughts on the ui part though for consideration:
1. Allow the labels "Available entities" and "Selected entities" to be altered in field edit setup to be more user friendly terminology for non-technical users.
2. Consider wrapping the list in div and adjusting css to 50% width for the container divs like example instead of 300px.

Example...

<div class="entityreference-dragdrop-widget-wrapper">
<div class="entityreference-dragdrop-container-available">
<div class="label"></div>
<ul></ul>
</div>
<div class="entityreference-dragdrop-container-selected">
<div class="label"></div>
<ul></ul>
</div>
</div>