env: environments can be named or created in directories

- `spack env create <name>` works as before

- `spack env create <path>` now works as well -- environments can be
  created in their own directories outside of Spack.

- `spack install` will look for a `spack.yaml` file in the current
  directory, and will install the entire project from the environment

- The Environment class has been refactored so that it does not depend on
  the internal Spack environment root; it just takes a path and operates
  on an environment in that path (so internal and external envs are
  handled the same)

- The named environment interface has been hoisted to the
  spack.environment module level.

- env.yaml is now spack.yaml in all places.  It was easier to go with one
  name for these files than to try to handle logic for both env.yaml and
  spack.yaml.

lib/spack/llnl/util/filesystem.py

@@ -538,6 +538,30 @@ def hash_directory(directory):
     return md5_hash.hexdigest()
 
 
+@contextmanager
+def write_tmp_and_move(filename):
+    """Write to a temporary file, then move into place."""
+    dirname = os.path.dirname(filename)
+    basename = os.path.basename(filename)
+    tmp = os.path.join(dirname, '.%s.tmp' % basename)
+    with open(tmp, 'w') as f:
+        yield f
+    shutil.move(tmp, filename)
+
+
+@contextmanager
+def open_if_filename(str_or_file, mode='r'):
+    """Takes either a path or a file object, and opens it if it is a path.
+
+    If it's a file object, just yields the file object.
+    """
+    if isinstance(str_or_file, six.string_types):
+        with open(str_or_file, mode) as f:
+            yield f
+    else:
+        yield str_or_file
+
+
 def touch(path):
     """Creates an empty file at the specified path."""
     perms = (os.O_WRONLY | os.O_CREAT | os.O_NONBLOCK | os.O_NOCTTY)