Removed readline dependency for ipython.terminal.console.completer

I noticed this issue while using the sublime repl plugin for ipython in sublime text, which used the `ZMQTerminalIPythonApp` class, which then imported `ZMQCompleter`, but then failed to run due to the readline dependency. I don't think this is necessary on windows, so I moved the import into the `rlcomplete` functions instead, which seems to fix the sublime repl issue.

I'm not very familar with ipython code, so please review, thanks!