Add Sphinx and its dependencies to requirements.

requirements.txt

+alabaster==0.7.7
+Babel==2.3.3
+docutils==0.12
+imagesize==0.7.0
+Jinja2==2.8
+MarkupSafe==0.23
 py==1.4.31
+Pygments==2.1.3
 pytest==2.9.1
+pytz==2016.3
+six==1.10.0
+snowballstemmer==1.2.1
+Sphinx==1.4.1
+sphinx-rtd-theme==0.1.9
 -e .