add cross-platform python deps to install_requires

Author: dessant

setup.py

@@ -1014,5 +1014,5 @@ def get_extensions_from_sources(sources):
         'Topic :: Software Development :: User Interfaces'],
     dependency_links=[
         'https://github.com/kivy-garden/garden/archive/master.zip'],
-    install_requires=['Kivy-Garden>=0.1.4'],
+    install_requires=['Kivy-Garden>=0.1.4', 'pillow>=3.1', 'docutils', 'pygments'],
     setup_requires=['cython>=' + MIN_CYTHON_STRING])