Stop beeping on every keypress

When the immodule is enabled, it currently causes a beep on every keypress. I think the problem is that it's passing unhandled commands to NSResponder's doCommandBySelector implementation, which beeps when it can't handle a command.

In GDB I set a breakpoint on NSBeep and found:

    Breakpoint 27, 0x96ffa3e1 in NSBeep ()
    (gdb) bt
    #0  0x96ffa3e1 in NSBeep ()
    #1  0x96fc52ba in -[NSResponder doCommandBySelector:] ()
    #2  0x0b9fc5d0 in -[GtkCocoaIMClient doCommandBySelector:] (self=0x83e125d0, _cmd=0x97935ddb, aSelector=0x97962526) at GtkCocoaIMClient.c:153
    #3  0x97056053 in -[NSTextInputContext doCommandBySelector:] ()
    #4  0x97055fcd in -[NSTextInputContext _handleCommand:] ()
    #5  0x97050999 in -[NSKeyBindingManager(NSKeyBindingManager_MultiClients) interpretEventAsCommand:forClient:] ()
    #6  0x9704fe25 in -[NSTextInputContext handleEvent:] ()
    #7  0x9704f895 in -[NSView interpretKeyEvents:] ()
    #8  0x0b9fbfc6 in -[GtkCocoaIMClient filterKeyDown:] (self=0x83e125d0, _cmd=0x6f141d5, keyDownEvent=0x8499ef80) at GtkCocoaIMClient.c:60
    #9  0x0b9fb767 in filter_keypress (context=0x7d0a8c38, event=0x7d485c18) at gtkimcontextcocoa.c:213
    #10 0x029c98c0 in gtk_im_context_filter_keypress (context=0x7d0a8c38, key=0x7d485c18) at gtkimcontext.c:473
    #11 0x029ce075 in gtk_im_multicontext_filter_keypress (context=0x7dbeaf30, event=0x7d485c18) at gtkimmulticontext.c:333
    #12 0x029c98c0 in gtk_im_context_filter_keypress (context=0x7dbeaf30, key=0x7d485c18) at gtkimcontext.c:473

etc.

And the `aSelector` being passed to the `doCommandBySelector:` is `moveDown:`:

    (gdb) p (char *) 0x97962526+8 
    $1 = 0x9796252e "moveDown:"

The [Apple docs](https://developer.apple.com/library/mac/#documentation/Cocoa/Reference/ApplicationKit/Protocols/NSTextInput_Protocol/Reference/Reference.html) say that although `doCommandBySelector:` is required on NSTextInputClient, it should not chain unhandled commands to its super NSResponder.