post URL is stored in temporary DB.

The post URL was part of the "meta" data that were originally
stored as a serialized object in the DB filed "meta".

In order to be able to query the DB using the URL (required
to handle reactions), URL is also stored in a separate DB col.
Reactions updates.

- bucket3 has new option "reactions".
- conf.example.yaml placeholders for twitter credentials, removed
hardcoded credentials from reactions.py.
- getTwitterReactions() will touch the coresponding (to the URL)
post file, so it will be re-rendered next time "bucket3 update"
is run.