[RFC] Make ForwardDiffNumbers subtypes of Real

[jrevels]

This PR resolve #66 by defining ForwardDiffNumber <: Real, and does some definition bookkeeping to resolve ambiguity warnings and get the tests to pass.

The most notable change (besides the titular one) is that some methods previously defined between Real and ForwardDiffNumber are now defined between ExternalReal and ForwardDiffNumber. ExternalReal is defined in ForwardDiff as:

julia> @eval typealias ExternalReal Union{$(subtypes(Real)...)}
Union{AbstractFloat,Integer,Irrational{sym},Rational{T<:Integer}}

This means that (in theory) custom T<:Real types should work, but only if they're defined before ForwardDiff is used/imported. It might be best to add "ForwardDiff doesn't support custom T<:Real types" in the docs to avoid this confusion all together. Thoughts? On a related note, it'd be helpful if someone else wanted to give the docs a once-over to make sure nothing else needs to be changed in the wake of this PR.

Additionally, this PR (once again, in theory) allows one to have Complex{T<:ForwardDiffNumber} types, so if anybody wants to play around with those, I'd be interested to see how it turns out. I don't think the API methods (gradient, hessian, etc.) will work with complex functions, but experiments that work directly with the number types should be feasible.

[mlubin]

I think it's fine to exclude custom real types until someone requests it.

[jrevels]

cc @Scidom, since this PR should help you with your Distributions.jl refactor (once it gets merged and tagged)

[papamarkou]

That's awesome, thanks a lot for doing all this work @jrevels. Yes, I will sort out the Distributions as promised once this gets merged. Moreover, I am nearly there with pushing the long-awaited changes to Lora at some point this month, so I will make sure it can work both with ForwardDiff and ReverseSourceDiff.

[papamarkou]

Shall we tag a new version and push to metadata? 👍

Before working on Distributions, I will try to integrate ForwardDiff with the new version of Lora, which should take me at most a week, as I am really curious how this would work :)