New: Added Pagerduty Integration (fixes #37)

fixes #37 

An overview of this PR:-
1. Supports live updating of incidents.
2. Post notes to pagerduty on adding comment.

Things left:
> If a user exists in both tools, an acknowledgement in PagerDuty assigns it to that user in banjaxed.

There is no support for other users in the app currently. How to go about it?
and tests.

To run:-
1. ```rake db:migrate```
2. Following env variables are used:- ```PAGERDUTY_API_KEY```, ```PAGERDUTY_SUBDOMAIN```, ```PAGERDUTY_REQUESTER_ID```.
2. Register on pagerduty and use the set subdomain for  ```PAGERDUTY_SUBDOMAIN```.
3. Go to https://```<subdomain>```.pagerduty.com/api_keys and create the api key for ```PAGERDUTY_API_KEY```.
4. Go to https://```<subdomain>```.pagerduty.com/users/```<user_id>```. Use ```<user_id>``` for ```PAGERDUTY_REQUESTER_ID```.
4. Add the callback:- https://```<app_domain>```.herokuapp.com/pagerduty/callback to pagerduty for webhook.
The `with: :exception` option here is important; it changes how invalid requests are handled, which can have security implications - without it you can no longer distinguish between a logged out user and a CSRF attack, for example.

Could you revert this change, and just skip the protection in the controller where it's necessary? As documented here: http://api.rubyonrails.org/classes/ActionController/RequestForgeryProtection/ClassMethods.html#method-i-protect_from_forgery