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 warn at error.jl:67
 in warn_once at util.jl:189
 in has at deprecated.jl:17
 in read at /Users/jameson/.julia/IniFile/src/IniFile.jl:42
 in read at /Users/jameson/.julia/IniFile/src/IniFile.jl:68
 in anonymous at /Users/jameson/.julia/Winston/src/Winston.jl:39
 in include_from_node1 at loading.jl:90
 in reload_path at loading.jl:113
 in require at loading.jl:48
```

Any concerns or should I go ahead and merge?