Switch to OAuth for reddit

 - remove the TODO
 - update the count method to use the stored token
    - also switch the iteration to a for loop—not sure why it wasn't to begin with
 - add an AccessTokenRetriever
 - changes to the config spec + create_section
Add TODO for workflowy OAuth
Fix using 'None' as the twitter API URL when custom URL not specified
Update for changes to Twython / twitter

 - Twython wants the callback_url passed to a different method
 - Twitter now requires the auth code
Switch to storing a personal access token for github

The OAuth flow breaks when the user has two-factor auth enabled.
Fixing it would mean finding adding support to github3.py, finding
a different library, or doing the auth manually.
Move the Google API config into the gmail section

Now that Google Reader is gone, there's no need for it to be
separate.
Switch to services being disabled-by-default

Enabled-by-default meant that a section being missing was
considered valid. This results in incorrect first-run behaviour.