Fixpy3

This fix the py3 Unicode error in the magics. Almost same fix as https://github.com/JuliaLang/pyjulia/pull/17, but I leverage the fact that we already have a compat module in IPython.

I also try-to fix the display of stack trace from the julia process. Though I wasn't able to get the stacktrace to string using the C api, and this code will probably not work on julia 0.2 but shoudl work on 0.3 as `jl_exception_occurred` seem to be in it.