This refactor makes the nested class 'StateHandler' in the
file /contrib_bots/bot_lib.py independent class. It previously
was nested in 'run_message_handler_for_bot' function.
This is done to write a cleaner test file for contrib_bots using
mock library.
To make all bots consistent add shared function in bot_lib.py
to check if this bot is called. All bots respond to at-mention of
the bot instead of their specific names.
Splitting out some of the bot functions into a library
will make it easier for heavily customized bots to have
their own version of run.py, instead of the shared one
that we use for everyone now. If they use bot_lib.py
directly, they will still most likely conform to
the "Handler" interface as long as they call
run_message_handler_for_bot.
Most bots should continue to use contrib_bots/run.py
for now.
