New, cleaner package repository structure.

Package repositories now look like this:

    top-level-dir/
        repo.yaml
        packages/
            libelf/
                package.py
            mpich/
                package.py
            ...

This leaves room at the top level for additional metadata, source,
per-repo configs, indexes, etc., and it makes it easy to see that
something is a spack repo (just look for repo.yaml and packages).

var/spack/repos/builtin/packages/tmuxinator/package.py

@@ -0,0 +1,17 @@
+from spack import *
+
+class Tmuxinator(Package):
+    """A session configuration creator and manager for tmux"""
+    homepage = "https://github.com/tmuxinator/tmuxinator"
+    url      = "https://github.com/tmuxinator/tmuxinator"
+
+    version('0.6.11', 
+        git='https://github.com/tmuxinator/tmuxinator',
+        tag='v0.6.11')
+
+    extends('ruby')
+
+    def install(self, spec, prefix):
+      gem('build', 'tmuxinator.gemspec')
+      gem('install', 'tmuxinator-{}.gem'.format(self.version))
+