Use single root for nif in WriteNifTree and ResolveMissingLinkStack, to make interface consistent with the rest of niflib.

Added regression test for the numTriangles issue reported by DragoonWraith (it fails at the moment).

  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.)