Update example commands from calling by keyword 'fs'
(As was done previously) to calling by at-mention of
the bot.
Updates '/contrib_bots/bots/virtual_fs/virtual_fs.py' by
correcting instructions on how to call 'virtual_fs' bot.
The previous test function in virtual_Bot made use of another function
called a 'sample_conversation' which had stateful example conversation
with virtual_fs bot.
The function sample_conversation is really useful, so made it accessible
to the users too.
Earlier, if virtual_fs bot is called without any argument, then the
bot returns nothing.
Now, virtual_fs bot replies with help message on how the user can
call this bot.
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.
In order to make the layout of all bots consistent, this commit
moves each bot into a folder with its name and modifies 'run.py'
so that only bots with such a structure can be executed. 'lib'
gets renamed to 'bots'.
