Use travis to check documentation.

.travis.yml

@@ -0,0 +1,11 @@
+# This is used to verify that the documentation compiles.
+language: python
+python:
+  - "2.7"
+# command to install dependencies
+install: "pip install -q Sphinx==1.1.3 --use-mirrors"
+# command to run tests
+script: cd docs && sphinx-build -nW -b html -d _build/doctrees . _build/html
+# Flags used here, not in `make html`:
+#  -n   Run in nit-picky mode. Currently, this generates warnings for all missing references.
+#  -W   Turn warnings into errors. This means that the build stops at the first warning and sphinx-build exits with exit status 1.