Tests for Variable and Constraint return values

[ccoffrin]

Once #9 is resolved, the number of elements returned from all variable_* calls should be exactly equal to the variables in the JuMP model. And a similar property should be true for the constraint_* calls.

A simple and generic way to test the return value implementation correctness is to test that the total number of returned constraints and variables are consistent with the internal JuMP model.

[ccoffrin]

Not longer relevant as of #158