ENH: Define extra dependencies for building docs

INSTALL.rst

@@ -156,6 +156,13 @@ Sphinx is required and a few extensions for it. Those can be installed as:
 
     pip install "sphinx>=1.2.3" sphinxcontrib-tikz sphinxcontrib-bayesnet sphinxcontrib-bibtex "numpydoc>=0.5"
 
+Or you can simply install BayesPy with ``doc`` extra, which will take care of
+installing the required dependencies:
+
+.. code-block:: console
+
+    pip install bayespy[doc]
+
 In order to visualize graphical models in HTML, you need to have ``ImageMagick``
 or ``Netpbm`` installed.  The documentation can be compiled to HTML and PDF by
 running the following commands in the ``doc`` directory:

setup.py

@@ -52,6 +52,15 @@ def read(fname):
             'matplotlib>=1.2.0',
             'h5py'
         ],
+        extras_require = {
+            'doc': [
+                'sphinx>=1.2.3',
+                'sphinxcontrib-tikz',
+                'sphinxcontrib-bayesnet',
+                'sphinxcontrib-bibtex',
+                'numpydoc>=0.5',
+            ],
+        },
         packages         = find_packages(),
         package_data     = {
             NAME: ["tests/baseline_images/test_plot/*.png"]