Implement AllowFiltering() LINQ clause on CqlQuery<T>

AllowFiltering was implemented only as a table attribute, but it is more related to a query. I added an extension method to the CqlQuery<TSource>, so that one can write LINQ queries like: 

<pre><code>
table
  .Where(item => item.SomeIndexedField == "x" && item.SomeOther == 1M)
  .AllowFiltering();
</code></pre>