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/postgresql/package.py

@@ -0,0 +1,20 @@
+from spack import *
+
+class Postgresql(Package):
+    """PostgreSQL is a powerful, open source object-relational
+       database system. It has more than 15 years of active
+       development and a proven architecture that has earned it a
+       strong reputation for reliability, data integrity, and
+       correctness."""
+    homepage = "http://www.postgresql.org/"
+    url      = "http://ftp.postgresql.org/pub/source/v9.3.4/postgresql-9.3.4.tar.bz2"
+
+    version('9.3.4', 'd0a41f54c377b2d2fab4a003b0dac762')
+
+    depends_on("openssl")
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix,
+                  "--with-openssl")
+        make()
+        make("install")