add update plugin

Calls git pull and bundle update before sending HUP to the bot
process. Requires the user that sends '!update' to be op of the channel.
use IO.popen instead of open3.popen3
use pm for detailed update messages
move reload to a seperate command
move raise StepError to invoke_command
remove require 'open3'
add send HUP to update command
Merge pull request #4 from relekang/update

add update plugin