add missing configuration for RTD sphinx theme

When using a packaged sphinx RTD theme we need to import the module (not
sure if this needs to be in a try for RTD proper), and include the path
so that sphinx can find the theme while building the documentation
during the installation.

doc/conf.py

@@ -13,6 +13,7 @@
 
 import sys, os
 import six
+import sphinx_rtd_theme
 
 # If extensions (or modules to document with autodoc) are in another directory,
 # add these directories to sys.path here. If the directory is relative to the
@@ -106,6 +107,7 @@
 
 # Add any paths that contain custom themes here, relative to this directory.
 #html_theme_path = []
+html_theme_path = [ sphinx_rtd_theme.get_html_theme_path() ]
 
 # The name for this set of Sphinx documents.  If None, it defaults to
 # "<project> v<release> documentation".