Adds _n() to po/build_pot.sh
_() now accepts format() arguments

We can now do:
    _(u"Hello {n}", n="Remi")

instead of:
    _(u"Hello {n}").format(n="Remi")

Note that positional arguments ("Hello {}") are NOT allowed, this is a
feature. Don't use positional arguments, word order can change anyway.
Uses _n() for removal confirmation