Explicitly check model/variable hygiene
JuMP has been pretty cavalier in tying Variables to Models; this
hopefully fixes that. Variables not owned by a model can be introduced
in the constraints, and printing will not choke on this. However, it
will spit an error when attempting to solve (this is important for
stochastic, where you want to share variables at the modeling---but
not solving---step.
Merge branch 'master' of https://github.com/JuliaOpt/JuMP.jl
Minor bugfix