gurobi and cplex use 1/2 coefficient in quad obj

Prior to v0.3.0b1, gurobi used v^T Q v and cplex used 1/2 v^T Q v.
In v0.3.0b1, this was switched to both using v^T Q v. However, the
convention appears to be to use the 1/2 (so what users will expect):
https://en.wikipedia.org/wiki/Quadratic_programming
Therefore, the 1/2 convention will be used for all solvers.