header and NiNode for now)

* created a common swig interface "common.i" to be used by all modules

* smart pointers now use the __call__() method, again this reduces the
  wrapper size

* removed __str__() from the niflib API as it does not belong there,
  instead renamed asString() to __str__() in the common.i interface

* version constants now also belong to the Niflib namespace

* SWIGFLAGS are now defined through scons environment, so we only need
  to set them once

* extra settings "-fcompact" and "-fvirtual" to SWIGFLAGS to reduce
  wrapper size

* enabled niflib as a shared library under non-win32 platforms
  (still need to find a solution for win32) under scons

* removed "pyniflib.h" and instead added CreateNiNode wrapper to ninode.i

* renamed "pyniflib.i" to "pyniflib.i_old" in case I forgot to port
  something to the new modularized system; to be removed in future

Changed order of %include's: Ref.h is now %included before niflib.h. It looks like the problem with illegal return types under SWIG is solved. (Solution suggested by Shon.)