Small ruby enhancement and tmuxinator package

It is currently less painful to pull the source from github, compile it into a
gem, then install the gem, than it is to download a gem and install it.  This
still lacks an activation mechanism, but `spack use tmuxinator` is functional.

lib/spack/spack/modules.py

@@ -130,6 +130,13 @@ def add_path(path_name, directory):
                 if site_packages:
                     add_path('PYTHONPATH', site_packages[0])
 
+            if self.spec.package.extends(spack.spec.Spec('ruby')):
+              add_path('GEM_PATH', self.spec.prefix)
+
+            # Allow module path extensions from packages
+            for key, val in self.spec.package.extra_environment:
+              add_path(key, val)
+
             # short description is just the package + version
             # TODO: maybe packages can optionally provide it.
             self.short_description = self.spec.format("$_ $@")