tox: add doc environment

Using tox to run Sphinx makes it easier to setup Jenkins since no
setup is needed: Jenkins already runs tox on all the default
environments.

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py27, py3, pep8, pyflakes, full
+envlist = py27, py3, pep8, pyflakes, full, doc
 
 [testenv]
 deps = -r{toxinidir}/test-requirements.txt
@@ -19,3 +19,9 @@ commands = pyflakes zpm setup.py zpmlib
 
 [testenv:full]
 commands = py.test --cov zpmlib --cov-report html --junit-xml junit.xml
+
+[testenv:doc]
+basepython = python
+changedir = doc
+deps = sphinx
+commands = sphinx-build -d _build/doctrees . _build/html