Remove references to py2exe.
Clarify Windows packaging instructions.
Update date in version number.
Use GtkSpell instead of pygtkspell.

GtkSpell seems to be in order again after the long silence amid the
arrival of GTK+3 and GObject introspection. GtkSpell is probably more
stable and easier to package. On Windows too, it works out of the box
with pygi-aio.