Added placeholder data attribute

This allows you to set a placeholder for attributes so there is always a visibly clickable area when the value is empty.

Example:

    <style type="text/css">
      .rest-in-placeholder {
        color: #AAA;
      }
    </style>
    ...
    <div data-url="<%= user_path(user) %>" data-object="user">
      <strong>User:</strong>
      <span class="rest-in-place" data-attribute="first_name" data-placeholder="First Name,">
        <%= user.first_name %>
      </span>
      <span class="rest-in-place" data-attribute="last_name" data-placeholder="Last Name">
        <%= user.last_name %>
      </span>
    </div>

![example](https://f.cloud.github.com/assets/34724/1981965/efad1cba-83eb-11e3-9468-7a8ef1ec1f37.png)