make empty _static directory "exist" to git

Sphinx really wants the _static directory to exist, but git doesn't.
This is a compromise to avoid superfluous warnings from sphinx.

See here:

http://stackoverflow.com/questions/115983/how-do-i-add-an-empty-directory-to-a-git-repository

lib/spack/docs/_static/.gitignore

+# Ignore everything in this directory
+*
+# Except this file
+!.gitignore