Update CI configuration: test install and test building docs.

.travis.yml

@@ -1,5 +1,10 @@
 language: python
 python:
   - "3.3"
-install: "pip install -r requirements.txt"
+install:
-script: "PYTHONPATH=. py.test"
+  - "pip install -r requirements.txt"
+  - "pip install sphinx"         # for test-building docs
+script:
+  - "PYTHONPATH=. py.test"       # run unit tests
+  - "python setup.py install"    # test install
+  - "sphinx-build docs html"     # test building docs