Use setuptools extras instead of requiring user to manually specify deps

docs/config.rst

@@ -137,6 +137,10 @@ protocol. Vdirsyncer contains **highly experimental support** for it.
 
     Do not use this storage if you're not prepared for data-loss and breakage.
 
+To use them, you need to install some optional dependencies with::
+
+    pip install vdirsyncer[remotestorage]
+
 .. autostorage:: vdirsyncer.storage.remotestorage.RemoteStorageContacts
 
 .. autostorage:: vdirsyncer.storage.remotestorage.RemoteStorageCalendars

docs/keyring.rst

@@ -44,7 +44,13 @@ System Keyring
 While the command approach is quite flexible, it is often cumbersome to write a
 script fetching the system keyring.
 
-Given that you have the keyring_ Python library installed, you can use::
+Vdirsyncer can do this for you if you have the keyring_ package installed. How
+you would obtain this package depends on how you installed vdirsyncer. If you
+used pip, you can use the following command to also install keyring::
+
+    pip install vdirsyncer[keyring]
+
+Then you can use::
 
     [storage foo]
     type = caldav

setup.py

@@ -42,5 +42,8 @@ setup(
         'requests_toolbelt>=0.4.0',
         'atomicwrites'
     ],
-    extras_require={'keyring': ['keyring']}
+    extras_require={
+        'keyring': ['keyring'],
+        'remotestorage': ['requests-oauthlib']
+    }
 )