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.
Why was this change necessary?