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/py-pandas/package.py

@@ -0,0 +1,25 @@
+from spack import *
+import os
+
+class PyPandas(Package):
+    """pandas is a Python package providing fast, flexible, and expressive data structures designed to make working with relational or labeled data both easy and intuitive. It aims to be the fundamental high-level building block for doing practical, real world data analysis in Python. Additionally, it has the broader goal of becoming the most powerful and flexible open source data analysis / manipulation tool available in any language."""
+    homepage = "http://pandas.pydata.org/"
+    url      = "https://pypi.python.org/packages/source/p/pandas/pandas-0.16.0.tar.gz#md5=bfe311f05dc0c351f8955fbd1e296e73"
+
+    version('0.16.0', 'bfe311f05dc0c351f8955fbd1e296e73')
+    version('0.16.1', 'fac4f25748f9610a3e00e765474bdea8')
+
+    extends('python')
+    depends_on('py-dateutil')
+    depends_on('py-numpy')
+    depends_on('py-matplotlib')
+    depends_on('py-scipy')
+    depends_on('py-setuptools')
+    depends_on('py-pytz')
+    depends_on('libdrm')
+    depends_on('libpciaccess')
+    depends_on('llvm')
+    depends_on('mesa')
+
+    def install(self, spec, prefix):
+        python('setup.py', 'install', '--prefix=%s' % prefix)