Added a REST API to List, Create, and Test rules

[CCoffie]

Below are some changes I made to ElastAlert that add a REST API. I added this because I've been working on a kibana plugin that I'll hopefully be releasing soon that will allow for quick and easy management of ElastAlert Rules from directly within kibana. Figured I'd push these now though because it might help other out there that want to do something similar.

[ikoniaris]

Cool idea! Also check your email @CCoffie :)

[viditmaniyar]

This is awesome! Great job! 👍

[thinairwireless]

This is a great addition and if combined with Backlog #420 would really bring a lot of power and flexibility to alerts. See untested fork here:
https://github.com/jagheterfredrik/elastalert/tree/rules_in_es

[kureus]

@Qmando Is there a roadmap for elastalert which shows which pull requests/issues are likely to be accepted? Would love to know if this and #420 are likely to make it in at any point?

[pataquets]

@Qmando : Would love to know the answer to @kureus question. I've found no 'contributing' doc or similar to answer it myself. If there isn't one, I can file a separate issue and bring some common used template as a PR to work on it, if you feel it valuable.

I'm interested on this REST feature, too since I am looking into adopting EA as a alerting solution and knowing such things would take out the uncertainity for us into investing into the tool and that would give peace of mind and clear the way to contributing, also.
The REST API would be an interesting feature for a big audience, IMO and might attract lots of interested developers, and I think it's sad of it being stalled.
@CCoffie : Are you still interested into working on this?