Adding a docs env to the tox project.

This will run the linkcheck and build the html docs, turning any warnings into
errors.
Also changed the theme to sphinx-doc and added version to be displayed in the
docs.

docs/check_sphinx.py

+import pytest
+import subprocess
+
+def test_linkcheck(tmpdir):
+    doctrees = tmpdir.join("doctrees")
+    htmldir = tmpdir.join("html")
+    subprocess.check_call(
+        ["sphinx-build", "-W", "-blinkcheck",
+          "-d", str(doctrees), ".", str(htmldir)])
+
+def test_build_docs(tmpdir):
+    doctrees = tmpdir.join("doctrees")
+    htmldir = tmpdir.join("html")
+    subprocess.check_call([
+        "sphinx-build", "-W", "-bhtml",
+          "-d", str(doctrees), ".", str(htmldir)])