Fix pretty printers about jump.jl

[IainNZ]

I've given up on compression of terms for quads, I would want to do it in a way that doesn't take n^2 memory, so need a different approach from the IndexedVector way (which was nice for making the affToStr issues)

[mlubin]

Goes gurobi accept duplicate quadratic terms? Compressing aff was important because solvers didn't like duplicate linear indices, but if gurobi doesn't care then it's not really an issue for now.

[IainNZ]

http://www.gurobi.com/documentation/5.5/reference-manual/node16
"The Gurobi interface is built around quadratic terms, rather than a Q matrix. If your quadratic objective contains a term 2 x y, you can enter it as a single term, 2 x y, or as a pair of terms, x y and y x."
Seems promising