[WIP] Julia v0.4 Compatibility 

I am working on getting OpenCL.jl to run with Julia v0.4 and this branch will track the changes necessary to do so.

Todo:
  - String -> AbstractString
  - Nothing -> Void
  - Seqfault for callbacks https://travis-ci.org/JuliaGPU/OpenCL.jl/jobs/44025578#L319-L330