Make call_on_each_{message,event} examples more clear

Before these examples weren't obviously blocking calls (they seemed
more like a callback registration, which may make more sense in the future)
trac: Include ticket summaries in body if they were truncated.
Add a Codebase integration
Move RSS and Twitter bots from demos/ to integrations/
Have Jira Groovy client send client=jira-groovy to our API.
Remove demos/ from setup.py install as it's no longer there
Release 0.2.1
humbug => zulip in log output
Rename humbug@humbughq.com to zulip@zulip.com.
git: Allow for the specification of a custom commit message formatter
codebase: Fix URLs
codebase: Truncate subjects to 60 characters
codebase: Add support for discussion comments
twitter-search-bot: Consistently capitalize Twitter.
twitter-search-bot: add the version requirement for twitter-python to the help.
twitter-search-bot: update the path to the bot in the help.
Add support for specifying client using zulip.init_from_options.
Use os.path directly rather than sometimes importing it separately.
Add Mercurial changegroup (hg push) integration.
Parse __version__ from __init__.py

This way we don't crash if dependencies aren't installed.

License assent:
    https://github.com/zulip/python-zulip/pull/3#issuecomment-18182458