Install sphinx with pip install -e .[docs] from now on

Removed the sphinx dependency as we don't need it when installing the package.

setup.py

@@ -50,6 +50,7 @@ setup(name='docxtpl',
       author_email='elapouya@gmail.com',
       license='LGPL 2.1',
       packages=['docxtpl'],
-      install_requires=['six', 'Sphinx<1.3b', 'sphinxcontrib-napoleon', 'python-docx', 'jinja2', 'lxml'],
+      install_requires=['six', 'python-docx', 'jinja2', 'lxml'],
+      extras_require={'docs': ['Sphinx<1.3b', 'sphinxcontrib-napoleon']},
       eager_resources=['docs'],
       zip_safe=False)