Implement wgetch

Thank you for encurses!  It should allow me to prototype games quickly in erlang. 

```getch``` is implemented to take keyboard input, but ```getch``` doesn't support windows.  An explanation is [here](http://stackoverflow.com/questions/19748685/curses-library-why-does-getch-clear-my-screen).

This patch implements ncurses' ```wgetch```, which is a general case of ```getch(0)```.

This escript illustrates the problem and the fix, depending on which line is enabled:

    #!/usr/bin/env escript
    %%! -noinput -pa ebin                                                                                                          

    -include_lib("include/encurses.hrl").

    main(_) ->
        encurses:initscr(),
        encurses:noecho(),
        encurses:curs_set(?CURS_INVISIBLE),

        Canvas = encurses:newwin(20,20,1,1),
        encurses:box(Canvas,?ACS_DIAMOND,?ACS_LANTERN),
        encurses:refresh(Canvas),
        Keyloop = fun F() -> 

                      %% Implicit refresh overwrites all windows                                                                   
                      %% Ch = encurses:getch(),                                                                                    

                      %% This allows input to windows                                                                              
                      Ch = encurses:getch(Canvas),

                      encurses:mvwaddstr(Canvas,10,10,"        "),
                      encurses:mvwaddstr(Canvas,10,10,io_lib:format("~p",[Ch])),
                      encurses:refresh(Canvas),
                      F()
              end,
        spawn(Keyloop),
        timer:sleep(10000),
        encurses:endwin().