add plugins dir (and related modif to Makefile) for "external applications"
add released wannier90 (from www.wannier90.org) in plugins
small modif exx example
small bug fixed in nscf calc save_in_cbands for compilation with -DEXX
git-svn-id: http://qeforge.qe-forge.org/svn/q-e/trunk/espresso@5927 c92efa57-630b-4861-b058-cf58834340f0