PR 66 rebased onto master

[cjprybol]

See https://github.com/JuliaData/DataTables.jl/pull/66. I opened this to avoid force pushing onto the old PR so we could preserve that as a reference in case I did anything wrong in the rebase.

Still todo:

• [ ] update CSV.jl with new DataStreams API and uncomment basic CSV file reading test in test/io.jl
• [ ] tag versions for and update REQUIRE with minimums for
  • [x] v0.0.6 Nulls.jl https://github.com/JuliaLang/METADATA.jl/pull/10979
  • [ ] v0.2.0 CategoricalArrays
  • [ ] v0.2.0 DataStreams https://github.com/JuliaLang/METADATA.jl/pull/10969 & https://github.com/JuliaLang/METADATA.jl/pull/10915
  • [ ] v0.2.0 CSV
  • [x] v0.3.0 WeakRefStrings https://github.com/JuliaLang/METADATA.jl/pull/10880