Handle GitHub redirections?

```
julia> contributors("dlfivefifty", "ApproxFun.jl")

HttpError(404,"Not Found","https://developer.github.com/v3")
while loading In[42], in expression starting on line 103

in handle_error at /Users/jiahao/.julia/v0.3/GitHub/src/error.jl:36
in get_pages at /Users/jiahao/.julia/v0.3/GitHub/src/utils.jl:78
in get_pages at /Users/jiahao/.julia/v0.3/GitHub/src/utils.jl:77
in contributors at /Users/jiahao/.julia/v0.3/GitHub/src/repos.jl:132
in contributors at /Users/jiahao/.julia/v0.3/GitHub/src/repos.jl:120
```

This particular package was moved to `https://github.com/ApproxFun/ApproxFun.jl`.

Start with the contributors path in repos.jl, then read get_pages in utils.jl and handle_error in error.jl using the reported ApproxFun.jl example. Reproduce the 404 for the moved repository and determine the expected redirect behavior; done means the contributors request handles that move successfully.

Written by the indexing model from the issue text.