Added "ne" and "null" tokens

Visual Studio 2013 has the following request when searching: 
/api/v2/Packages()?$filter=(((Id%20ne%20null)%20and%20substringof('sync',tolower(Id)))%20or%20((Description%20ne%20null)%20and%20substringof('sync',tolower(Description))))%20or%20((Tags%20ne%20null)%20and%20substringof('%20sync%20',tolower(Tags)))


This now works