Qualify special functions with module name.

[tpapp]

Fixes #17.

Lines were broken because they became too long.

[tpapp]

Someone knowledgeable please review the line

@define_diffrule NaNMath.lgamma(x) = :(  SpecialFunctions.digamma($x)               )

because I am not sure about the corner cases.

[jrevels]

I assume that that's fine, but I'll ask @mlubin since his packages - e.g. JuMP - are the main users of these definitions.

[mlubin]

Actually JuMP doesn't currently use DiffRules, although we'd like to. But anyway is the issue that digamma is missing from NaNMath?

[tpapp]

The issue is that

ForwardDiff.derivative(NaNMath.lgamma, x)

may be nonsensical for x where NaNMath.lgamma differs from SpecialFunctions.lgamma, since it uses SpecialFunctioncs.digamma. But it may just work if the analytic extension matches. I don't know enough about the gamma function to answer this.

In any case, unless there are objections, it would be great to merge this PR. It fixes some bugs, and I don't think it makes anything worse.

[mlubin]

I don't know enough about lgamma or digamma either, so no objection here.

[andreasnoack]

Would be great to get this one in since it is holding back downstream packages.

[jrevels]

Actually JuMP doesn't currently use DiffRules, although we'd like to.

JuMP uses ForwardDiff, which uses DiffRules. But anyway, this seems fine. Thanks again @tpapp!