make the API pure UTF-8

This is probably the more reasonable change that makes everything UTF-8 and addresses https://github.com/quinnj/ODBC.jl/issues/67 for me. However, the question is whether this will work with all drivers or if some of them *require* UTF-16. @jakebolewski, perhaps you can chime in since it seems like you may have introduced some of the UTF-16 stuff that I'm now removing.