make replpane work with julia master

Or is it some outstanding patch for julia master I am missing? (`REPL.mode_keymap` and `keymap_merge` without the bang seems to be missing)

Btw,  really interesting stuff. When this is more ready perhaps the backend part of this could be reused to make a C++ kernel for Jupyter/IPython, or perhaps a C++ "cell magic" for  the IJulia kernel.