Add default instantiation of a Model

[mlubin]

See discussion at #762 (comment).

Motivation:
"I feel I'm casting spells rather than writing code at the moment" (jump-dev/SCS.jl#145 (comment))

[blegat]

A good example is given in https://github.com/JuliaOpt/MathOptInterface.jl/blob/master/test/model.jl