Add invoke task for serving documentation.

Less typing, and a bit more obvious for anyone else.

tasks.py

@@ -8,6 +8,11 @@ def test():
 def doc():
    run('cd docs && make html')
 
+@task
+def docserve():
+   print 'Serving docs on localhost:8000...'
+   run('cd docs/_build/html && python -m SimpleHTTPServer')
+
 @task
 def publish():
    run('./setup.py sdist upload')