Use staged functions in ``@variable``

[joehuchette]

Currently we don't recognize this as something we can store as a JuMPArray:

I = 1:3
@defVar(m, x[I])

It should be fairly straightforward to add a staged function in the generated macro code that takes the JuMPArray branch based on the type of I.

[mlubin]

As I alluded to in #337 we can't easily implement this until we move over to the dormant JuMPArray.jl implementation because we're currently generating custom types inside of the macros. That's sketchy enough as it is, I don't think we should try to generate types at runtime within staged functions.

[joehuchette]

Should we try to bump the ntuple inlining thing?

[mlubin]

Not sure if it will be helpful, it's already a well-known issue.

[mlubin]

Don't think this is needed for JuMP 0.10, it'll be easier to do after we drop support for 0.3.