Add tox test for docs

And ignore the warning about changelog_archive, it's really not meant to
be in the toctree.

docs/changelog_archive.rst

@@ -1,3 +1,5 @@
+:orphan:
+
 Archive of old changelog entries
 ================================
 

tox.ini

@@ -6,6 +6,7 @@ envlist=
     py27-django15,
     py26-django14,
     py27-django14,
+    docs,
 
 [testenv]
 commands=
@@ -43,3 +44,7 @@ basepython=python3.2
 deps=
     django==1.5
 
+[testenv:docs]
+changedir=docs
+deps=Sphinx
+commands=sphinx-build -W -b html -d {envtmpdir}/doctrees . {envtmpdir}/html