add backtrace to warn and info

This adds backtraces to warn and info to make it easier to track down the offending line of code. e.g:
```
julia> using Winston
WARNING: has(d,k) is deprecated, use haskey(d,k) instead.
 in read at /Users/jameson/.julia/IniFile/src/IniFile.jl:42
```

Any concerns or should I go ahead and merge?