[Julia API] Use Ref instead of length-1 Vector for error code and some minor stylistic changes

First commit changes Array{Cint}(1) to Ref{Cint}() for error codes. This is is faster and allocates less memory. Second commit has no functional changes, but includes some changes of style. I realize this might be subjective, but what I changed to here is the style used in most julia packages, and the style we use in Julia base. Happy to drop this commit if you prefer the current style.