lmod : added support for the creation of hierarchical lua module files (#1723)

Includes :
- treatment of a generic hierarchy (i.e. lapack + mpi + compiler)
- possibility to specify which compilers are to be considered Core
- correct treatment of the 'family' directive
- unit tests for most new features

.gitignore

@@ -12,6 +12,7 @@
 /etc/spackconfig
 /share/spack/dotkit
 /share/spack/modules
+/share/spack/lmod
 /TAGS
 /htmlcov
 .coverage