Rishabh & Parth 2012 xapian-letor work

Refactoring on which subsequent GSoC work is based.

Also includes implementations of ListNet & ListMLE which have since been discarded due to possible licensing conflicts (this is noted in the source code in these commits).
That's one broken copy constructor.  Better to declare it private and no implement it at all if you're not going to implement it to do anything.