`gamma(0)` returns the wrong value.

This issue is the opposite issue of https://github.com/JuliaMath/SpecialFunctions.jl/issues/163. Currently `gamma(0.0)` returns `Inf` as does `gamma(0.0+0.0im)`, and `gamma(0.0-0.0im)` This isn't the correct limit because
```
gamma(0. + 1e-16im)
0.6123233995736787 - 1.0000000000000034e16im
```
and
```
julia> gamma(0.0-1e-16im)
0.6123233995736787 + 1.0000000000000034e16im
```
which really makes it seem like `gamma` of complex 0s should return an appropriate complex infinity and `gamma(0.0)` should error.