Update for new heuristic callback design

@carlobaldassi 

I've just implemented heuristic callbacks in JuMP. To provide maximum flexibility, I had to change a couple of things. Because Gurobi can support partially completed solutions, and because specifying the values of all variables is annoying, I added a new method to the interface. Basically, you set the solution of variables, and submit the solution to the solver seperately. Now, because GLPK doesn't support partial solutions in that way, I default the solution vector to the current node solution, which is probably all you need most of the time.

Please review and adjust as you see fit. I'll tag this PR in the main JuMP.jl PR - you can follow it so see all the other PRs.