Add keybindings for live coding

Oi, 
thanks for epresent it is the no-thrills approach to presentations that I was looking for.

I saw you worked on refreshing the buffer when code was executed but I was unable to execute code (C-c C-c din't work when inside epresent-mode), nor was I able to modify it (C-c ') . So I just added some key bindings

    e for edit the source block
    x for executing it

The only thing that is missing is that when editing code inside present-mode the cursor is still blank, any pointers on how to fix that?