(CFACT-137) Fix Unicode support

Command-line arguments aren't necessarily passed in Unicode, Boost.Log and
Boost.Filesystem use wchar_t instead of UTF-8 by default, and Ruby uses
local encoding by default. Change all of these to use UTF-8 in char.
`rb_utf8_encoding` appears to have a fairly lightweight implementation once the encoding table is initialized.  Although `api` currently has a singleton interface, if we somehow, in the future, support different ruby VMs in process, I'd prefer it if we didn't store values statically across `api` instances.  What are your thoughts about changing this to just pass in `rb_utf8_encoding()` to `rb_enc_str_new`?