Add support for modeling with SI units

Something that has been mentioned before in passing and might be worth discussing down the track: allow modeling with SI units using https://github.com/ajkeller34/Unitful.jl. It may be fairly trivial, I'm not sure.

DifferentialEquations.jl seems to allow this, for example [here](https://github.com/JuliaDiffEq/DiffEqTutorials.jl/blob/719c21cd76d8133b9f6a62ecceb10dfcccd24123/ExtraODEFeatures/.ipynb_checkpoints/Unit%20Checked%20Arithmetic%20via%20Unitful-checkpoint.ipynb).

GPKit is a geometric programming toolkit in Python and it [supports specification of units](https://gpkit.readthedocs.io/en/latest/gettingstarted.html?highlight=units) and automatically does dimensional analysis to convert or verify units in expressions/constraints. From the GPKit docs, an example of a variable declaration:
```julia
# Declare a variable, z, with units of meters, and a description
z = Variable("z", "m", "A variable called z with units of meters")
```
and a simple model:
```julia
# consider a block with dimensions x, y, z less than 1
# constrain surface area less than 1.0 m^2
x = Variable("x", "m")
y = Variable("y", "m")
z = Variable("z", "m")
S = Variable("S", 1.0, "m^2")
c = (2*x*y + 2*x*z + 2*y*z <= S)
```
(There are other ideas we could borrow from GPKit to make JuMP even more user friendly. I like the little solution tables it can print after a solve.)