Function renaming and deprecation from #517

This covers more of what was proposed in #517:
* Stop exporting functions already exposed by the included `using DataArrays, StatsBase`
* Stop exporting `read_rda` helpers
* Remove functions that had been copied to `deprecated.jl` from original files
* Move more deprecated functions to `deprecated.jl`
* More of the proposed deprecations
* More of the proposed renaming

There still isn't great symmetry in the IO function names, but nothing came to mind:

save(filename, df) | loaddf(filename)
--- | ---
writetable(filename, df) | readtable(filename)