bitbucket ips and suggestions for script

I have been using this to pull from bitbucket -- thanks for starting this project :heart: 

I added bitbucket IPs -- I also had to make changes in the code to ignore subnet origin.  The payload would need to be inspected differently.  I thought I would just submit this pull request to see if you are interested in perhaps cleaning up this module some and making it open to easily accept hooks from other services (perhaps some kind of config files to describe each service to describe the ips, subnets, and data to read in the request)

This also has a suggestion for using fetch/reset instead of pull (merge) in the script.