Fix OS X source packaging, add PackageMaker generator to create installer, rename LICENSE -> LICENSE.txt to keep PackageMaker happy

Generalised Cmake SUBDIRS inclusions in solvers and PreProcessing to only
include available directories.
Added scripts to generate tarball of code.


git-svn-id: https://gforge.sci.utah.edu/svn/nektar/trunk@2455 305cdda6-5ce1-45b3-a98d-dfc68c8b3305